🕷️ Crawler Inspector

URL Lookup

Direct Parameter Lookup

Raw Queries and Responses

1. Shard Calculation

Query:

Response:

Calculated Shard: 40 (from laksa053)

2. Crawled Status Check

Query:

curl -X POST \
  'http://laksa040.int.ahrefs:8124/' \
  -H 'Content-Type: text/plain' \
  -H 'X-ClickHouse-Database: crawler3' \
  -H 'Authorization: Basic YXBpOg==' \
  -d 'SELECT getAhrefsURLFromUnparsed(src_unparsed) AS found_url, ifNull(toUnixTimestamp(download_stamp), 0) AS crawl_time, ifNull(toUnixTimestamp(props_url_first_seen), 0) AS first_indexed_time, download_http_code AS http_code, src_unparsed AS src_unparsed, src_root_hash AS src_root_hash, history_drop_reason AS history_drop_reason, meta_title AS meta_title, meta_descriptions AS meta_descriptions, attrs_boilerpipe_text AS attrs_boilerpipe_text, attrs_markdown AS attrs_markdown, attrs_readable_markdown AS attrs_readable_markdown, meta_canonical AS meta_canonical FROM crawler3.page_info_local FINAL PREWHERE (src_root_hash, src_unparsed) IN ((getAhrefsRootHashFromUnparsed(getAhrefsUnparsedNoserviceFromURL(\'https://www.w3schools.com/dsa/dsa_data_queues.php\')), getAhrefsUnparsedNoserviceFromURL(\'https://www.w3schools.com/dsa/dsa_data_queues.php\'))) FORMAT JSONEachRow'

Response:

{"found_url":"https:\/\/www.w3schools.com\/dsa\/dsa_data_queues.php","crawl_time":1775265043,"first_indexed_time":1706651218,"http_code":200,"src_unparsed":"com,w3schools!www,\/dsa\/dsa_data_queues.php s443","src_root_hash":"10756103332767711440","history_drop_reason":null,"meta_title":"DSA Queues","meta_descriptions":["Well organized and easy to understand Web building tutorials with lots of examples of how to use HTML, CSS, JavaScript, SQL, PHP, Python, Bootstrap, Java and XML."],"attrs_boilerpipe_text":"DSA\nQueues\nQueues\nA queue is a data structure that can hold many elements.\n3\n2\n1\n4\nResult:\nThink of a queue as people standing in line in a supermarket.\nThe first person to stand in line is also the first who can pay and leave the supermarket. This way of organizing elements is called FIFO: First In First Out.\nBasic operations we can do on a queue are:\nEnqueue:\nAdds a new element to the queue.\nDequeue:\nRemoves and returns the first (front) element from the queue.\nPeek:\nReturns the first element in the queue.\nisEmpty:\nChecks if the queue is empty.\nSize:\nFinds the number of elements in the queue.\nExperiment with these basic operations in the queue animation above.\nQueues can be implemented by using arrays or linked lists.\nQueues can be used to implement job scheduling for an office printer, order processing for e-tickets, or to create algorithms for breadth-first search in graphs.\nQueues are often mentioned together with Stacks, which is a similar data structure described on the\nprevious page\n.\nQueue Implementation using Arrays\nTo better understand the benefits with using arrays or linked lists to implement queues, you should check out\nthis page\nthat explains how arrays and linked lists are stored in memory.\nThis is how it looks like when we use an array as a queue:\n[\n3\n,\n2\n,\n2\n,\n2\n]\nResult:\nReasons to implement queues using arrays:\nMemory Efficient:\nArray elements do not hold the next elements address like linked list nodes do.\nEasier to implement and understand:\nUsing arrays to implement queues require less code than using linked lists, and for this reason it is typically easier to understand as well.\nReasons for\nnot\nusing arrays to implement queues:\nFixed size:\nAn array occupies a fixed part of the memory. This means that it could take up more memory than needed, or if the array fills up, it cannot hold more elements. And resizing an array can be costly.\nShifting cost:\nDequeue causes the first element in a queue to be removed, and the other elements must be shifted to take the removed elements' place. This is inefficient and can cause problems, especially if the queue is long.\nAlternatives:\nSome programming languages have built-in data structures optimized for queue operations that are better than using arrays.\nNote:\nWhen using arrays in Python for this tutorial, we are really using the Python 'list' data type, but for the scope of this tutorial the 'list' data type can be used in the same way as an array. Learn more about Python lists\nhere\n.\nSince Python lists has good support for functionality needed to implement queues, we start with creating a queue and do queue operations with just a few lines:\nExample\nPython:\nqueue\n=\n[\n]\n# Enqueue\nqueue\n.\nappend\n(\n'A'\n)\nqueue\n.\nappend\n(\n'B'\n)\nqueue\n.\nappend\n(\n'C'\n)\nprint\n(\n\"Queue: \"\n,\nqueue\n)\n# Dequeue\nelement\n=\nqueue\n.\npop\n(\n0\n)\nprint\n(\n\"Dequeue: \"\n,\nelement\n)\n# Peek\nfrontElement\n=\nqueue\n[\n0\n]\nprint\n(\n\"Peek: \"\n,\nfrontElement\n)\n# isEmpty\nisEmpty\n=\nnot\nbool\n(\nqueue\n)\nprint\n(\n\"isEmpty: \"\n,\nisEmpty\n)\n# Size\nprint\n(\n\"Size: \"\n,\nlen\n(\nqueue\n)\n)\nTry it Yourself »\nBut to explicitly create a data structure for queues, with basic operations, we should create a queue class instead. This way of creating queues in Python is also more similar to how queues can be created in other programming languages like C and Java.\nExample\nPython:\nclass\nQueue\n:\ndef\n__init__\n(\nself\n)\n:\nself\n.\nqueue\n=\n[\n]\ndef\nenqueue\n(\nself\n,\nelement\n)\n:\nself\n.\nqueue\n.\nappend\n(\nelement\n)\ndef\ndequeue\n(\nself\n)\n:\nif\nself\n.\nisEmpty\n(\n)\n:\nreturn\n\"Queue is empty\"\nreturn\nself\n.\nqueue\n.\npop\n(\n0\n)\ndef\npeek\n(\nself\n)\n:\nif\nself\n.\nisEmpty\n(\n)\n:\nreturn\n\"Queue is empty\"\nreturn\nself\n.\nqueue\n[\n0\n]\ndef\nisEmpty\n(\nself\n)\n:\nreturn\nlen\n(\nself\n.\nqueue\n)\n==\n0\ndef\nsize\n(\nself\n)\n:\nreturn\nlen\n(\nself\n.\nqueue\n)\n# Create a queue\nmyQueue\n=\nQueue\n(\n)\nmyQueue\n.\nenqueue\n(\n'A'\n)\nmyQueue\n.\nenqueue\n(\n'B'\n)\nmyQueue\n.\nenqueue\n(\n'C'\n)\nprint\n(\n\"Queue: \"\n,\nmyQueue\n.\nqueue\n)\nprint\n(\n\"Dequeue: \"\n,\nmyQueue\n.\ndequeue\n(\n)\n)\nprint\n(\n\"Peek: \"\n,\nmyQueue\n.\npeek\n(\n)\n)\nprint\n(\n\"isEmpty: \"\n,\nmyQueue\n.\nisEmpty\n(\n)\n)\nprint\n(\n\"Size: \"\n,\nmyQueue\n.\nsize\n(\n)\n)\nTry it Yourself »\nQueue Implementation using Linked Lists\nReasons for using linked lists to implement queues:\nDynamic size:\nThe queue can grow and shrink dynamically, unlike with arrays.\nNo shifting:\nThe front element of the queue can be removed (dequeue) without having to shift other elements in the memory.\nReasons for\nnot\nusing linked lists to implement queues:\nExtra memory:\nEach queue element must contain the address to the next element (the next linked list node).\nReadability:\nThe code might be harder to read and write for some because it is longer and more complex.\nThis is how a queue can be implemented using a linked list.\nExample\nPython:\nclass\nNode\n:\ndef\n__init__\n(\nself\n,\ndata\n)\n:\nself\n.\ndata\n=\ndata\n        self\n.\nnext\n=\nNone\nclass\nQueue\n:\ndef\n__init__\n(\nself\n)\n:\nself\n.\nfront\n=\nNone\nself\n.\nrear\n=\nNone\nself\n.\nlength\n=\n0\ndef\nenqueue\n(\nself\n,\nelement\n)\n:\nnew_node\n=\nNode\n(\nelement\n)\nif\nself\n.\nrear\nis\nNone\n:\nself\n.\nfront\n=\nself\n.\nrear\n=\nnew_node\n            self\n.\nlength\n+=\n1\nreturn\nself\n.\nrear\n.\nnext\n=\nnew_node\n        self\n.\nrear\n=\nnew_node\n        self\n.\nlength\n+=\n1\ndef\ndequeue\n(\nself\n)\n:\nif\nself\n.\nisEmpty\n(\n)\n:\nreturn\n\"Queue is empty\"\ntemp\n=\nself\n.\nfront\n        self\n.\nfront\n=\ntemp\n.\nnext\nself\n.\nlength\n-=\n1\nif\nself\n.\nfront\nis\nNone\n:\nself\n.\nrear\n=\nNone\nreturn\ntemp\n.\ndata\ndef\npeek\n(\nself\n)\n:\nif\nself\n.\nisEmpty\n(\n)\n:\nreturn\n\"Queue is empty\"\nreturn\nself\n.\nfront\n.\ndata\ndef\nisEmpty\n(\nself\n)\n:\nreturn\nself\n.\nlength\n==\n0\ndef\nsize\n(\nself\n)\n:\nreturn\nself\n.\nlength\ndef\nprintQueue\n(\nself\n)\n:\ntemp\n=\nself\n.\nfront\nwhile\ntemp\n:\nprint\n(\ntemp\n.\ndata\n,\nend\n=\n\" \"\n)\ntemp\n=\ntemp\n.\nnext\nprint\n(\n)\n# Create a queue\nmyQueue\n=\nQueue\n(\n)\nmyQueue\n.\nenqueue\n(\n'A'\n)\nmyQueue\n.\nenqueue\n(\n'B'\n)\nmyQueue\n.\nenqueue\n(\n'C'\n)\nprint\n(\n\"Queue: \"\n,\nend\n=\n\"\"\n)\nmyQueue\n.\nprintQueue\n(\n)\nprint\n(\n\"Dequeue: \"\n,\nmyQueue\n.\ndequeue\n(\n)\n)\nprint\n(\n\"Peek: \"\n,\nmyQueue\n.\npeek\n(\n)\n)\nprint\n(\n\"isEmpty: \"\n,\nmyQueue\n.\nisEmpty\n(\n)\n)\nprint\n(\n\"Size: \"\n,\nmyQueue\n.\nsize\n(\n)\n)\nTry it Yourself »","attrs_markdown":"[Tutorials](\"Tutorials and References\")   [References](\"References\")   [Exercises](\"Exercises and Quizzes\")   [Certificates](\"Certificates\")\n\n[Menu](\"Menu\")\n\nSearch field\n\n*×*\n\n[See More]()\n\nSign 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[Reference](https:\/\/www.w3schools.com\/vue\/vue_ref_builtin-attributes.php \"Vue Reference\")\n\n[Learn Angular](https:\/\/www.w3schools.com\/angular\/default.asp \"Angular Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/angular\/default.asp \"Angular Tutorial\")\n\n[Learn AngularJS](https:\/\/www.w3schools.com\/angularjs\/default.asp \"AngularJS Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/angularjs\/default.asp \"AngularJS Tutorial\")   [Reference](https:\/\/www.w3schools.com\/angularjs\/angularjs_ref_directives.asp \"AngularJS Reference\")\n\n[Learn JSON](https:\/\/www.w3schools.com\/js\/js_json_intro.asp \"JSON Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/js\/js_json_intro.asp \"JSON Tutorial\")   [Reference](https:\/\/www.w3schools.com\/jsref\/jsref_obj_json.asp \"JSON Reference\")\n\n[Learn AJAX](https:\/\/www.w3schools.com\/js\/js_ajax_intro.asp \"AJAX Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/js\/js_ajax_intro.asp \"AJAX Tutorial\")\n\n[Learn AppML](https:\/\/www.w3schools.com\/appml\/default.asp \"AppML Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/appml\/default.asp \"AppML Tutorial\")   [Reference](https:\/\/www.w3schools.com\/appml\/appml_reference.asp \"AppML Reference\")\n\n[Learn W3.JS](https:\/\/www.w3schools.com\/w3js\/default.asp \"W3.JS Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/w3js\/default.asp \"W3.JS Tutorial\")   [Reference](https:\/\/www.w3schools.com\/w3js\/w3js_references.asp \"W3.JS Reference\")\n\n### Web Building\n[Create a Website](https:\/\/www.w3schools.com\/spaces\/index.php \"Get Your Own Website With W3schools Spaces\")\n\n[Create a Server NEW](https:\/\/www.w3schools.com\/spaces\/index.php \"Get Your Own Server With W3schools Spaces\")\n\n[Where To Start](https:\/\/www.w3schools.com\/where_to_start.asp \"Where To Start - Web Development\")\n\n[Web Templates](https:\/\/www.w3schools.com\/w3css\/w3css_templates.asp \"Free Web Templates\")\n\n[Web Statistics](https:\/\/www.w3schools.com\/browsers\/default.asp \"Web Statistics\")\n\n[Web Certificates](https:\/\/campus.w3schools.com\/ \"Certificates\")\n\n[Web Development](https:\/\/www.w3schools.com\/whatis\/default.asp \"Web Development Roadmaps\")\n\n[Introduction to Programming](https:\/\/www.w3schools.com\/programming\/index.php \"Introduction to Programming\")\n\n[Introduction to HTML & CSS](https:\/\/www.w3schools.com\/htmlcss\/default.asp \"HTML & CSS\")\n\n[Code Editor](https:\/\/www.w3schools.com\/tryit\/default.asp \"Try it - Code Editor\")\n\n[Test Your Typing Speed](https:\/\/www.w3schools.com\/typingspeed\/default.asp \"Test Your Typing Speed\")\n\n[Play a Code Game](https:\/\/www.w3schools.com\/codegame\/index.html \"Play a Code Game\")\n\n[Web Developer Tools](https:\/\/www.w3schools.com\/tools\/index.php \"Web Developer Tools\")\n\n[Cyber Security](https:\/\/www.w3schools.com\/cybersecurity\/index.php \"Cyber Security Tutorial\")\n\n[Accessibility](https:\/\/www.w3schools.com\/accessibility\/index.php \"Accessibility Security Tutorial\")\n\n[Join our Newsletter](https:\/\/campus.w3schools.com\/pages\/newsletter \"W3Schools Newsletter\")\n\n### Backend\n[Learn Python](https:\/\/www.w3schools.com\/python\/default.asp \"Python Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/python\/default.asp \"Python Tutorial\")   [Reference](https:\/\/www.w3schools.com\/python\/python_reference.asp \"Python Reference\")\n\n[Learn SQL](https:\/\/www.w3schools.com\/sql\/default.asp \"SQL Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/sql\/default.asp \"SQL Tutorial\")   [Reference](https:\/\/www.w3schools.com\/sql\/sql_ref_keywords.asp \"SQL Reference\")\n\n[Learn MySQL](https:\/\/www.w3schools.com\/mysql\/default.asp \"MySQL Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/mysql\/default.asp \"MySQL Tutorial\")   [Reference](https:\/\/www.w3schools.com\/mysql\/mysql_datatypes.asp \"MySQL Reference\")\n\n[Learn PHP](https:\/\/www.w3schools.com\/php\/default.asp \"PHP Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/php\/default.asp \"PHP Tutorial\")   [Reference](https:\/\/www.w3schools.com\/php\/php_ref_overview.asp \"PHP Reference\")\n\n[Learn Java](https:\/\/www.w3schools.com\/java\/default.asp \"Java Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/java\/default.asp \"Java Tutorial\")   [Reference](https:\/\/www.w3schools.com\/java\/java_ref_reference.asp \"Java Reference\")\n\n[Learn C](https:\/\/www.w3schools.com\/c\/index.php \"C Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/c\/index.php \"C Tutorial\")   [Reference](https:\/\/www.w3schools.com\/c\/c_ref_reference.php \"C Reference\")\n\n[Learn C++](https:\/\/www.w3schools.com\/cpp\/default.asp \"C++ Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/cpp\/default.asp \"C++ Tutorial\")   [Reference](https:\/\/www.w3schools.com\/cpp\/cpp_ref_reference.asp \"C++ Reference\")\n\n[Learn C\\#](https:\/\/www.w3schools.com\/cs\/index.php \"C# Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/cs\/index.php \"C# Tutorial\")\n\n[Learn R](https:\/\/www.w3schools.com\/r\/default.asp \"R Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/r\/default.asp \"R Tutorial\")\n\n[Learn Kotlin](https:\/\/www.w3schools.com\/kotlin\/index.php \"Kotlin Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/kotlin\/index.php \"Kotlin Tutorial\")\n\n[Learn Rust](https:\/\/www.w3schools.com\/rust\/index.php \"Rust Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/rust\/index.php \"Rust Tutorial\")\n\n[Learn Go](https:\/\/www.w3schools.com\/go\/index.php \"Go Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/go\/index.php \"Go Tutorial\")\n\n[Learn Django](https:\/\/www.w3schools.com\/django\/index.php \"Django Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/django\/index.php \"Django Tutorial\")   [Reference](https:\/\/www.w3schools.com\/django\/django_ref_template_tags.php \"Django Reference\")\n\n[Learn PostgreSQL](https:\/\/www.w3schools.com\/postgresql\/index.php \"PostgreSQL Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/postgresql\/index.php \"PostgreSQL Tutorial\")\n\n[Learn TypeScript](https:\/\/www.w3schools.com\/typescript\/index.php \"TypeScript Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/typescript\/index.php \"TypeScript Reference\")\n\n[Learn ASP](https:\/\/www.w3schools.com\/asp\/default.asp \"ASP Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/asp\/default.asp \"ASP Tutorial\")   [Reference](https:\/\/www.w3schools.com\/asp\/asp_ref_vbscript_functions.asp \"ASP Reference\")\n\n[Learn Node.js](https:\/\/www.w3schools.com\/nodejs\/default.asp \"Node.js Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/nodejs\/default.asp \"Node.js Tutorial\")   [Reference](https:\/\/www.w3schools.com\/nodejs\/ref_modules.asp \"Node.js Reference\")\n\n[Learn Raspberry Pi](https:\/\/www.w3schools.com\/nodejs\/nodejs_raspberrypi.asp \"Raspberry Pi Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/nodejs\/nodejs_raspberrypi.asp \"Raspberry Pi Tutorial\")\n\n[Learn Swift](https:\/\/www.w3schools.com\/swift\/default.asp \"Swift Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/swift\/default.asp \"Swift Tutorial\")\n\n[Learn Git](https:\/\/www.w3schools.com\/git\/default.asp \"Git Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/git\/default.asp \"Git Tutorial\")\n\n[Learn Bash](https:\/\/www.w3schools.com\/bash\/index.php \"Bash Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/bash\/index.php \"Bash Tutorial\")\n\n[Learn MongoDB](https:\/\/www.w3schools.com\/mongodb\/index.php \"MongoDB Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/mongodb\/index.php \"MongoDB Tutorial\")\n\n[Learn AWS Cloud](https:\/\/www.w3schools.com\/aws\/index.php \"AWS Cloud Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/aws\/index.php \"AWS Cloud Tutorial\")\n\n[Learn XML](https:\/\/www.w3schools.com\/xml\/default.asp \"XML Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/xml\/default.asp \"XML Tutorial\")   [Reference](https:\/\/www.w3schools.com\/xml\/dom_nodetype.asp \"XML Reference\")\n\n### Data Analytics\n[Learn AI](https:\/\/www.w3schools.com\/ai\/default.asp \"Artificial Intelligence Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/ai\/default.asp \"Artificial Intelligence Tutorial\")\n\n[Learn Generative AI](https:\/\/www.w3schools.com\/gen_ai\/index.php \"Generative AI Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/gen_ai\/index.php \"Generative AI Tutorial\")\n\n[Learn ChatGPT-3.5](https:\/\/www.w3schools.com\/gen_ai\/chatgpt-3-5\/index.php \"ChatGPT-3.5 Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/gen_ai\/chatgpt-3-5\/index.php \"ChatGPT-3.5  Tutorial\")\n\n[Learn ChatGPT-4](https:\/\/www.w3schools.com\/gen_ai\/chatgpt-4\/index.php \"ChatGPT-4 Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/gen_ai\/chatgpt-4\/index.php \"ChatGPT-4 Tutorial\")\n\n[Learn Google Bard](https:\/\/www.w3schools.com\/gen_ai\/bard\/index.php \"Google Bard Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/gen_ai\/bard\/index.php \"Google Bard Tutorial\")\n\n[Learn Machine Learning](https:\/\/www.w3schools.com\/python\/python_ml_getting_started.asp \"Machine Learning Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/python\/python_ml_getting_started.asp \"Machine Learning Tutorial\")\n\n[Learn DSA](https:\/\/www.w3schools.com\/dsa\/index.php \"DSA - Data Structures and Algorithms\")   [Tutorial](https:\/\/www.w3schools.com\/dsa\/index.php \"DSA - Data Structures and Algorithms \")\n\n[Learn Data Science](https:\/\/www.w3schools.com\/datascience\/default.asp \"Data Science Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/datascience\/default.asp \"Data Science Tutorial\")\n\n[Learn NumPy](https:\/\/www.w3schools.com\/python\/numpy\/default.asp \"NumPy Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/python\/numpy\/default.asp \"NumPy Tutorial\")\n\n[Learn Pandas](https:\/\/www.w3schools.com\/python\/pandas\/default.asp \"Pandas Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/python\/pandas\/default.asp \"Pandas Tutorial\")\n\n[Learn SciPy](https:\/\/www.w3schools.com\/python\/scipy\/index.php \"SciPy Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/python\/scipy\/index.php \"SciPy Tutorial\")\n\n[Learn Matplotlib](https:\/\/www.w3schools.com\/python\/matplotlib_intro.asp \"Matplotlib Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/python\/matplotlib_intro.asp \"Matplotlib Tutorial\")\n\n[Learn Statistics](https:\/\/www.w3schools.com\/statistics\/index.php \"Statistics Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/statistics\/index.php \"Statistics Tutorial\")\n\n[Learn Excel](https:\/\/www.w3schools.com\/excel\/index.php \"Excel Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/excel\/index.php \"Excel Tutorial\")\n\n[Learn Google Sheets](https:\/\/www.w3schools.com\/googlesheets\/index.php \"Google Sheets Tutorial\")   [Tutorial](https:\/\/www.w3schools.com\/googlesheets\/index.php \"Google Sheets Tutorial\")\n\n### Web Building\n[Create a Website](https:\/\/www.w3schools.com\/spaces\/index.php \"Get Your Own Website With W3schools Spaces\")\n\n[Create a Server NEW](https:\/\/www.w3schools.com\/spaces\/index.php \"Get Your Own Server With W3schools Spaces\")\n\n[Where To Start](https:\/\/www.w3schools.com\/where_to_start.asp \"Where To Start - Web Development\")\n\n[Web Templates](https:\/\/www.w3schools.com\/w3css\/w3css_templates.asp \"Free Web Templates\")\n\n[Web Statistics](https:\/\/www.w3schools.com\/browsers\/default.asp \"Web Statistics\")\n\n[Web Certificates](https:\/\/campus.w3schools.com\/ \"Certificates\")\n\n[Web Development](https:\/\/www.w3schools.com\/whatis\/default.asp \"Web Development Roadmaps\")\n\n[Introduction to Programming](https:\/\/www.w3schools.com\/programming\/index.php \"Introduction to Programming\")\n\n[Introduction to HTML & CSS](https:\/\/www.w3schools.com\/htmlcss\/default.asp \"HTML & CSS\")\n\n[Code Editor](https:\/\/www.w3schools.com\/tryit\/default.asp \"Try it - Code Editor\")\n\n[Test Your Typing Speed](https:\/\/www.w3schools.com\/typingspeed\/default.asp \"Test Your Typing Speed\")\n\n[Play a Code Game](https:\/\/www.w3schools.com\/codegame\/index.html \"Play a Code Game\")\n\n[Web Developer Tools](https:\/\/www.w3schools.com\/tools\/index.php \"Web Developer Tools\")\n\n[Cyber Security](https:\/\/www.w3schools.com\/cybersecurity\/index.php \"Cyber Security Tutorial\")\n\n[Accessibility](https:\/\/www.w3schools.com\/accessibility\/index.php \"Accessibility Security Tutorial\")\n\n[Join our Newsletter](https:\/\/campus.w3schools.com\/pages\/newsletter \"W3Schools Newsletter\")\n\n×\n\n## **References**\nReferences filter input\n\n×\n\n### HTML and CSS\n[HTML Tags Reference](https:\/\/www.w3schools.com\/tags\/default.asp \"HTML Tag Reference\")\n\n[CSS Reference](https:\/\/www.w3schools.com\/cssref\/default.asp \"CSS Reference\")\n\n[W3.CSS Reference](https:\/\/www.w3schools.com\/w3css\/w3css_references.asp \"W3.CSS Reference\")\n\n[Bootstrap 3 Reference](https:\/\/www.w3schools.com\/bootstrap\/bootstrap_ref_all_classes.asp \"Bootstrap3 Reference\")\n\n[Bootstrap 4 Reference](https:\/\/www.w3schools.com\/bootstrap4\/bootstrap_ref_all_classes.asp \"Bootstrap4 Reference\")\n\n[Color Names](https:\/\/www.w3schools.com\/colors\/colors_names.asp \"Color Names\")\n\n[Icons Reference](https:\/\/www.w3schools.com\/icons\/default.asp \"Icons Reference\")\n\n[SVG Reference](https:\/\/www.w3schools.com\/graphics\/svg_reference.asp \"SVG Reference\")\n\n[Canvas Reference](https:\/\/www.w3schools.com\/graphics\/canvas_reference.asp \"Canvas Reference\")\n\n[Sass Reference](https:\/\/www.w3schools.com\/sass\/sass_functions_string.php \"SASS Reference\")\n\n[UTF-8 Charset Reference](https:\/\/www.w3schools.com\/charsets\/default.asp \"UTF-8 Reference\")\n\n[UTF-8 Emojis Reference](https:\/\/www.w3schools.com\/charsets\/ref_emoji_smileys.asp \"UTF-8 Emojis Reference\")\n\n### JavaScript\n[JavaScript Reference](https:\/\/www.w3schools.com\/jsref\/default.asp \"JavaScript Reference\")\n\n[HTML DOM Reference](https:\/\/www.w3schools.com\/jsref\/dom_obj_document.asp \"HTML DOM Reference\")\n\n[jQuery Reference](https:\/\/www.w3schools.com\/jquery\/jquery_ref_overview.asp \"jQuery Reference\")\n\n[Vue Reference](https:\/\/www.w3schools.com\/vue\/vue_ref_builtin-attributes.php \"Vue Reference\")\n\n[Angular Reference](https:\/\/www.w3schools.com\/angular\/angularjs_ref_directives.asp \"Angular Reference\")\n\n[JSON Reference](https:\/\/www.w3schools.com\/jsref\/jsref_obj_json.asp \"JSON Reference\")\n\n[AppML Reference](https:\/\/www.w3schools.com\/appml\/appml_reference.asp \"AppML Reference\")\n\n[W3.JS Reference](https:\/\/www.w3schools.com\/e3js\/w3js_references.asp \"W3.JS Reference\")\n\n### Backend\n[Python Reference](https:\/\/www.w3schools.com\/python\/python_reference.asp \"Python Reference\")\n\n[SQL Reference](https:\/\/www.w3schools.com\/sql\/sql_ref_keywords.asp \"SQL Reference\")\n\n[MySQL Reference](https:\/\/www.w3schools.com\/mysql\/mysql_datatypes.asp \"MySQL Reference\")\n\n[PHP Reference](https:\/\/www.w3schools.com\/php\/php_ref_overview.asp \"PHP Reference\")\n\n[Java Reference](https:\/\/www.w3schools.com\/java\/java_ref_reference.asp \"Java Reference\")\n\n[C Reference](https:\/\/www.w3schools.com\/c\/c_ref_reference.php \"C Reference\")\n\n[C++ Reference](https:\/\/www.w3schools.com\/cpp\/cpp_ref_reference.asp \"C++ Reference\")\n\n[Django Reference](https:\/\/www.w3schools.com\/django\/django_ref_template_tags.php \"Django Reference\")\n\n[ASP Reference](https:\/\/www.w3schools.com\/asp\/asp_ref_vbscript_functions.asp \"ASP Reference\")\n\n[Node.js Reference](https:\/\/www.w3schools.com\/nodejs\/ref_modules.asp \"Node.js Reference\")\n\n[XML Reference](https:\/\/www.w3schools.com\/xml\/dom_nodetype.asp \"XML Reference\")\n\n×\n\n## **Exercises**\nExcercises filter input\n\n×\n\n### HTML and CSS\n[HTML](https:\/\/www.w3schools.com\/html\/html_exercises.asp \"HTML Exercises\")   [Exercise](https:\/\/www.w3schools.com\/html\/html_exercises.asp \"HTML Exercises\")   [Quiz](https:\/\/www.w3schools.com\/html\/html_quiz.asp \"HTML Quizzes\")   [Challenge](https:\/\/www.w3schools.com\/html\/html_challenges.asp \"HTML Code Challenges\")\n\n[CSS](https:\/\/www.w3schools.com\/css\/css_exercises.asp \"CSS Exercises\")   [Exercise](https:\/\/www.w3schools.com\/css\/css_exercises.asp \"CSS Exercises\")   [Quiz](https:\/\/www.w3schools.com\/css\/css_quiz.asp \"CSS Quizzes\")   [Challenge](https:\/\/www.w3schools.com\/css\/css_challenges.asp \"CSS Code Challenges\")\n\n[Bootstrap 3](https:\/\/www.w3schools.com\/bootstrap\/bootstrap_exercises.asp \"Bootstrap 3 Exercises\")   [Exercise](https:\/\/www.w3schools.com\/bootstrap\/bootstrap_exercises.asp \"Bootstrap 3 Exercises\")   [Quiz](https:\/\/www.w3schools.com\/bootstrap\/bootstrap_quiz.asp \"Bootstrap 3 Quizzes\")\n\n[Bootstrap 4](https:\/\/www.w3schools.com\/bootstrap4\/bootstrap_exercises.asp \"Bootstrap 4 Exercises\")   [Exercise](https:\/\/www.w3schools.com\/bootstrap4\/bootstrap_exercises.asp \"Bootstrap 4 Exercises\")   [Quiz](https:\/\/www.w3schools.com\/bootstrap4\/bootstrap_quiz.asp \"Bootstrap 4 Quizzes\")\n\n[Bootstrap 5](https:\/\/www.w3schools.com\/bootstrap5\/bootstrap_exercises.php \"Bootstrap 5 Exercises\")   [Exercise](https:\/\/www.w3schools.com\/bootstrap5\/bootstrap_exercises.php \"Bootstrap 5 Exercises\")   [Quiz](https:\/\/www.w3schools.com\/bootstrap5\/bootstrap_quiz.php \"Bootstrap 5 Quizzes\")\n\n### Data Analytics\n[DSA](https:\/\/www.w3schools.com\/dsa\/dsa_exercises.php \"DSA Exercises\")   [Exercise](https:\/\/www.w3schools.com\/dsa\/dsa_exercises.php \"DSA Exercises\")   [Quiz](https:\/\/www.w3schools.com\/dsa\/dsa_quiz.php \"DSA Quizzes\")\n\n[NumPy](https:\/\/www.w3schools.com\/python\/numpy\/numpy_exercises.asp \"NumPy Exercises\")   [Exercise](https:\/\/www.w3schools.com\/python\/numpy\/numpy_exercises.asp \"NumPy Exercises\")   [Quiz](https:\/\/www.w3schools.com\/python\/numpy\/numpy_quiz.asp \"NumPy Quizzes\")\n\n[Pandas](https:\/\/www.w3schools.com\/python\/pandas\/pandas_exercises.asp \"Pandas Exercises\")   [Exercise](https:\/\/www.w3schools.com\/python\/pandas\/pandas_exercises.asp \"Pandas Exercises\")   [Quiz](https:\/\/www.w3schools.com\/python\/pandas\/pandas_quiz.asp \"Pandas Quizzes\")\n\n[SciPy](https:\/\/www.w3schools.com\/python\/scipy\/scipy_exercises.php \"SciPy Exercises\")   [Exercise](https:\/\/www.w3schools.com\/python\/scipy\/scipy_exercises.php \"SciPy Exercises\")   [Quiz](https:\/\/www.w3schools.com\/python\/scipy\/scipy_quiz.php \"SciPy Quizzes\")\n\n[Excel](https:\/\/www.w3schools.com\/excel\/excel_exercises.php \"Excel Exercises\")   [Exercise](https:\/\/www.w3schools.com\/excel\/excel_exercises.php \"Excel Exercises\")\n\n[What is an Exercise?](https:\/\/www.w3schools.com\/exercises\/index.php \"W3Schools Exercises\")   [What is a Quiz?](https:\/\/www.w3schools.com\/quiztest\/default.asp \"W3Schools Quizzes\")   [What is a Challenge?](https:\/\/www.w3schools.com\/challenges\/index.php \"W3Schools Challenges\")\n\n### JavaScript\n[JavaScript](https:\/\/www.w3schools.com\/js\/js_exercises.asp \"JavaScript Exercises\")   [Exercise](https:\/\/www.w3schools.com\/js\/js_exercises.asp \"JavaScript Exercises\")   [Quiz](https:\/\/www.w3schools.com\/js\/js_quiz.asp \"JavaScript Quizzes\")\n\n[React](https:\/\/www.w3schools.com\/react\/react_exercises.asp \"React Exercises\")   [Exercise](https:\/\/www.w3schools.com\/react\/react_exercises.asp \"React Exercises\")   [Quiz](https:\/\/www.w3schools.com\/react\/react_quiz.asp \"React Quizzes\")\n\n[jQuery](https:\/\/www.w3schools.com\/jquery\/jquery_exercises.asp \"jQuery Exercises\")   [Exercise](https:\/\/www.w3schools.com\/jquery\/jquery_exercises.asp \"jQuery Exercises\")   [Quiz](https:\/\/www.w3schools.com\/jquery\/jquery_quiz.asp \"jQuery Quizzes\")\n\n[Vue](https:\/\/www.w3schools.com\/vue\/vue_exercises.php \"Vue Exercises\")   [Exercise](https:\/\/www.w3schools.com\/vue\/vue_exercises.php \"Vue Exercises\")   [Quiz](https:\/\/www.w3schools.com\/vue\/vue_quiz.php \"Vue Quizzes\")\n\n[Angular](https:\/\/www.w3schools.com\/angular\/angular_exercises.asp \"Angular Exercises\")   [Exercise](https:\/\/www.w3schools.com\/angular\/angular_exercises.asp \"Angular Exercises\")   [Quiz](https:\/\/www.w3schools.com\/angular\/angular_quiz.asp \"Angular Quizzes\")\n\n### Backend\n[Python](https:\/\/www.w3schools.com\/python\/python_exercises.asp \"Python Exercises\")   [Exercise](https:\/\/www.w3schools.com\/python\/python_exercises.asp \"Python Exercises\")   [Quiz](https:\/\/www.w3schools.com\/python\/python_quiz.asp \"Python Quizzes\")   [Challenge](https:\/\/www.w3schools.com\/python\/python_challenges.asp \"Python Code Challenges\")\n\n[SQL](https:\/\/www.w3schools.com\/sql\/sql_exercises.asp \"SQL Exercises\")   [Exercise](https:\/\/www.w3schools.com\/sql\/sql_exercises.asp \"SQL Exercises\")   [Quiz](https:\/\/www.w3schools.com\/sql\/sql_quiz.asp \"SQL Quizzes\")\n\n[MySQL](https:\/\/www.w3schools.com\/mysql\/mysql_exercises.asp \"MySQL Exercises\")   [Exercise](https:\/\/www.w3schools.com\/mysql\/mysql_exercises.asp \"MySQL Exercises\")   [Quiz](https:\/\/www.w3schools.com\/mysql\/mysql_quiz.asp \"MySQL Quizzes\")\n\n[PHP](https:\/\/www.w3schools.com\/php\/php_exercises.asp \"PHP Exercises\")   [Exercise](https:\/\/www.w3schools.com\/php\/php_exercises.asp \"PHP Exercises\")   [Quiz](https:\/\/www.w3schools.com\/php\/php_quiz.asp \"PHP Quizzes\")\n\n[Java](https:\/\/www.w3schools.com\/java\/java_exercises.asp \"Java Exercises\")   [Exercise](https:\/\/www.w3schools.com\/java\/java_exercises.asp \"Java Exercises\")   [Quiz](https:\/\/www.w3schools.com\/java\/java_quiz.asp \"Java Quizzes\")   [Challenge](https:\/\/www.w3schools.com\/java\/java_challenges.asp \"Java Code Challenges\")\n\n[C](https:\/\/www.w3schools.com\/c\/c_exercises.php \"C Exercises\")   [Exercise](https:\/\/www.w3schools.com\/c\/c_exercises.php \"C Exercises\")   [Quiz](https:\/\/www.w3schools.com\/c\/c_quiz.php \"C Quizzes\")   [Challenge](https:\/\/www.w3schools.com\/c\/c_challenges.php \"C Challenges\")\n\n[C++](https:\/\/www.w3schools.com\/cpp\/cpp_exercises.asp \"C++ Exercises\")   [Exercise](https:\/\/www.w3schools.com\/cpp\/cpp_exercises.asp \"C++ Exercises\")   [Quiz](https:\/\/www.w3schools.com\/cpp\/cpp_quiz.asp \"C++ Quizzes\")   [Challenge](https:\/\/www.w3schools.com\/cpp\/cpp_challenges.asp \"C++ Code Challenges\")\n\n[C\\#](https:\/\/www.w3schools.com\/cs\/cs_exercises.php \"C# Exercises\")   [Exercise](https:\/\/www.w3schools.com\/cs\/cs_exercises.php \"C# Exercises\")   [Quiz](https:\/\/www.w3schools.com\/cs\/cs_quiz.php \"C# Quizzes\")\n\n[R](https:\/\/www.w3schools.com\/r\/r_exercises.asp \"R Exercises\")   [Exercise](https:\/\/www.w3schools.com\/r\/r_exercises.asp \"R Exercises\")   [Quiz](https:\/\/www.w3schools.com\/r\/r_quiz.asp \"R Quizzes\")\n\n[Kotlin](https:\/\/www.w3schools.com\/kotlin\/kotlin_exercises.php \"Kotlin Exercises\")   [Exercise](https:\/\/www.w3schools.com\/kotlin\/kotlin_exercises.php \"Kotlin Exercises\")   [Quiz](https:\/\/www.w3schools.com\/kotlin\/kotlin_quiz.php \"Kotlin Quizzes\")\n\n[Django](https:\/\/www.w3schools.com\/django\/django_exercises.php \"Django Exercises\")   [Exercise](https:\/\/www.w3schools.com\/django\/django_exercises.php \"Django Exercises\")   [Quiz](https:\/\/www.w3schools.com\/django\/django_quiz.php \"Django Quizzes\")\n\n[Node.js](https:\/\/www.w3schools.com\/nodejs\/nodejs_exercises.asp \"Node.js Exercises\")   [Exercise](https:\/\/www.w3schools.com\/nodejs\/nodejs_exercises.asp \"Node.js Exercises\")   [Quiz](https:\/\/www.w3schools.com\/nodejs\/nodejs_quiz.asp \"Node.js Quizzes\")\n\n[PostgreSQL](https:\/\/www.w3schools.com\/postgresql\/postgresql_exercises.php \"PostgreSQL Exercises\")   [Exercise](https:\/\/www.w3schools.com\/postgresql\/postgresql_exercises.php \"PostgreSQL Exercises\")   [Quiz](https:\/\/www.w3schools.com\/postgresql\/postgresql_quiz.php \"PostgreSQL Quizzes\")\n\n[TypeScript](https:\/\/www.w3schools.com\/typescript\/typescript_exercises.php \"TypeScript Exercises\")   [Exercise](https:\/\/www.w3schools.com\/typescript\/typescript_exercises.php \"TypeScript Exercises\")   [Quiz](https:\/\/www.w3schools.com\/typescript\/typescript_quiz.php \"TypeScript Quizzes\")\n\n[Git](https:\/\/www.w3schools.com\/git\/git_exercises.asp \"Git Exercises\")   [Exercise](https:\/\/www.w3schools.com\/git\/git_exercises.asp \"Git Exercises\")   [Quiz](https:\/\/www.w3schools.com\/git\/git_quiz.asp \"Git Quizzes\")\n\n[Bash](https:\/\/www.w3schools.com\/bash\/bash_exercises.php \"Bash Exercises\")   [Exercise](https:\/\/www.w3schools.com\/bash\/bash_exercises.php \"Bash Exercises\")   [Quiz](https:\/\/www.w3schools.com\/bash\/bash_quiz.php \"Bash Quizzes\")\n\n[Go](https:\/\/www.w3schools.com\/go\/go_exercises.php \"Go Exercises\")   [Exercise](https:\/\/www.w3schools.com\/go\/go_exercises.php \"Go Exercises\")\n\n[MongoDB](https:\/\/www.w3schools.com\/mongodb\/mongodb_exercises.php \"MongoDB Exercises\")   [Exercise](https:\/\/www.w3schools.com\/mongodb\/mongodb_exercises.php \"MongoDB Exercises\")\n\n### Data Analytics\n[DSA](https:\/\/www.w3schools.com\/dsa\/dsa_exercises.php \"DSA Exercises\")   [Exercise](https:\/\/www.w3schools.com\/dsa\/dsa_exercises.php \"DSA Exercises\")   [Quiz](https:\/\/www.w3schools.com\/dsa\/dsa_quiz.php \"DSA Quizzes\")\n\n[NumPy](https:\/\/www.w3schools.com\/python\/numpy\/numpy_exercises.asp \"NumPy Exercises\")   [Exercise](https:\/\/www.w3schools.com\/python\/numpy\/numpy_exercises.asp \"NumPy Exercises\")   [Quiz](https:\/\/www.w3schools.com\/python\/numpy\/numpy_quiz.asp \"NumPy Quizzes\")\n\n[Pandas](https:\/\/www.w3schools.com\/python\/pandas\/pandas_exercises.asp \"Pandas Exercises\")   [Exercise](https:\/\/www.w3schools.com\/python\/pandas\/pandas_exercises.asp \"Pandas Exercises\")   [Quiz](https:\/\/www.w3schools.com\/python\/pandas\/pandas_quiz.asp \"Pandas Quizzes\")\n\n[SciPy](https:\/\/www.w3schools.com\/python\/scipy\/scipy_exercises.php \"SciPy Exercises\")   [Exercise](https:\/\/www.w3schools.com\/python\/scipy\/scipy_exercises.php \"SciPy Exercises\")   [Quiz](https:\/\/www.w3schools.com\/python\/scipy\/scipy_quiz.php \"SciPy Quizzes\")\n\n[Excel](https:\/\/www.w3schools.com\/excel\/excel_exercises.php \"Excel Exercises\")   [Exercise](https:\/\/www.w3schools.com\/excel\/excel_exercises.php \"Excel Exercises\")\n\n[What is an Exercise?](https:\/\/www.w3schools.com\/exercises\/index.php \"W3Schools Exercises\")   [What is a Quiz?](https:\/\/www.w3schools.com\/quiztest\/default.asp \"W3Schools Quizzes\")   [What is a Code Challenge?](https:\/\/www.w3schools.com\/challenges\/index.php \"W3Schools Code Challenges\")\n\n×\n\n## **Certificates**\nFilter field for certifications\n\n×\n\n### HTML and CSS\n[HTML](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/html-certificate \"HTML Certification Exam\")   [Certificate](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/html-certificate \"HTML Certification Exam\")   [Course](https:\/\/campus.w3schools.com\/collections\/course-catalog\/products\/html-course \"Paid HTML Course\")   [Bootcamp](https:\/\/www.w3schools.com\/bootcamp\/html-css.php \"HTML Bootcamp\")\n\n[CSS](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/css-certificate \"CSS Certification Exam\")   [Certificate](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/css-certificate \"CSS Certification Exam\")   [Course](https:\/\/campus.w3schools.com\/collections\/course-catalog\/products\/css-course \"Paid CSS Course\")   [Bootcamp](https:\/\/www.w3schools.com\/bootcamp\/html-css.php \"CSS Bootcamp\")\n\n[Bootstrap 3](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/bootstrap-3-certificate \"Bootstrap 3 Certification Exam\")   [Certificate](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/bootstrap-3-certificate \"Bootstrap 3 Certification Exam\")   [Course](https:\/\/campus.w3schools.com\/collections\/single-courses\/products\/bootstrap-course \"Paid Bootstrap 3 Course\")\n\n[Bootstrap 4](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/bootstrap-4-certificate \"Bootstrap 4 Certification Exam\")   [Certificate](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/bootstrap-4-certificate \"Bootstrap 4 Certification Exam\")   [Course](https:\/\/campus.w3schools.com\/collections\/single-courses\/products\/bootstrap-4-course \"Paid Bootstrap 4 Course\")\n\n[Bootstrap 5](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/bootstrap-5-certificate \"Bootstrap 5 Certification Exam\")   [Certificate](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/bootstrap-5-certificate \"Bootstrap 5 Certification Exam\")\n\n### Data Analytics\n[DSA](https:\/\/campus.w3schools.com\/products\/dsa-certification-exam \"DSA Certificate Exam\")   [Certificate](https:\/\/campus.w3schools.com\/products\/dsa-certification-exam \"DSA Certificate Exam\")\n\n[Data Analytics](https:\/\/campus.w3schools.com\/products\/data-analytics-program \"Paid Data Analytics Course\")   [Course](https:\/\/campus.w3schools.com\/products\/data-analytics-program \"Paid Data Analytics Course\")\n\n[NumPy](https:\/\/campus.w3schools.com\/products\/numpy-certification-exam \"NumPy Certification Exam\")   [Certificate](https:\/\/campus.w3schools.com\/products\/numpy-certification-exam \"NumPy Certification Exam\")   [Course](https:\/\/campus.w3schools.com\/products\/numpy-course \"Paid NumPy Course\")\n\n[Pandas](https:\/\/campus.w3schools.com\/products\/pandas-certification-exam \"Pandas Certification Exam\")   [Certificate](https:\/\/campus.w3schools.com\/products\/pandas-certification-exam \"Pandas Certification Exam\")   [Course](https:\/\/campus.w3schools.com\/products\/pandas-course \"Paid Pandas Course\")\n\n[Excel](https:\/\/campus.w3schools.com\/products\/excel-certificate \"Excel Certification Exam\")   [Certificate](https:\/\/campus.w3schools.com\/products\/excel-certificate \"Excel Certification Exam\")\n\n[Social Media](https:\/\/campus.w3schools.com\/collections\/course-best-sellers\/products\/social-media-marketing-course \"Paid Social Media Course\")   [Course](https:\/\/campus.w3schools.com\/collections\/course-best-sellers\/products\/social-media-marketing-course \"Paid Social Media Course\")\n\n[What is a Certificate?](https:\/\/campus.w3schools.com\/ \"W3Schools Campus\")\n\n[What is a Bootcamp?](https:\/\/www.w3schools.com\/bootcamp\/index.php \"W3Schools Bootcamps\")\n\n### Programs\n[Full Access Best Value\\!](https:\/\/campus.w3schools.com\/collections\/course-catalog\/products\/w3schools-full-access-course \"Paid Full Access Course\")\n\n[Front End](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/front-end-course \"Paid Front End Course\")   [Certificate](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/front-end-certificate \"Front End Certification Exam\")   [Course](https:\/\/campus.w3schools.com\/collections\/course-catalog\/products\/front-end-course \"Paid Front End Development Course\")\n\n[Web Dev.](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/modern-web-development-certification \"Web Development Certification\")   [Certificate](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/modern-web-development-certification \"Web Development Certification\")   [Course](https:\/\/campus.w3schools.com\/collections\/course-best-sellers\/products\/learn-modern-web-development \"Paid Web Development Course\")\n\n[Web App](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/web-application-development-certificates \"Web Application Development Certification\")   [Certificate](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/web-application-development-certificates \"Web Application Development Certification\")   [Course](https:\/\/campus.w3schools.com\/collections\/course-best-sellers\/products\/web-application-development-course \"Paid Web Application Course\")\n\n[Web Design](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/web-design-certification \"Web Design Certification Exam\")   [Certificate](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/web-design-certification \"Web Design Certification Exam\")   [Course](https:\/\/campus.w3schools.com\/collections\/course-best-sellers\/products\/learn-web-design \"Paid Web Design Course\")\n\n### JavaScript\n[JavaScript](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/javascript-certificate \"JavaScript Certification Exam\")   [Certificate](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/javascript-certificate \"JavaScript Certification Exam\")   [Course](https:\/\/campus.w3schools.com\/collections\/course-catalog\/products\/javascript-course \"Paid JavaScript Course\")   [Bootcamp](https:\/\/www.w3schools.com\/bootcamp\/javascript.php \"JavaScript Bootcamp\")\n\n[React](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/react-js-certificate \"React.js Certification Exam\")   [Certificate](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/react-js-certificate \"React.js Certification Exam\")   [Course](https:\/\/campus.w3schools.com\/collections\/course-catalog\/products\/react-js-course \"Paid React.js Course\")   [Bootcamp](https:\/\/www.w3schools.com\/bootcamp\/react.php \"React Bootcamp\")\n\n[jQuery](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/jquery-certificate \"jQuery Certification Exam\")   [Certificate](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/jquery-certificate \"jQuery Certification Exam\")   [Course](https:\/\/campus.w3schools.com\/collections\/course-catalog\/products\/jquery-course \"Paid jQuery Course\")\n\n[Vue](https:\/\/campus.w3schools.com\/products\/vue-js-certification-exam \"Vue Certification Exam\")   [Certificate](https:\/\/campus.w3schools.com\/products\/vue-js-certification-exam \"Vue Certification Exam\")\n\n### Programs\n[Full Access Best Value\\!](https:\/\/campus.w3schools.com\/collections\/course-catalog\/products\/w3schools-full-access-course \"Paid Full Access Course\")\n\n[Front End](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/front-end-certificate \"Front End Certification Exam\")   [Certificate](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/front-end-certificate \"Front End Certification Exam\")   [Course](https:\/\/campus.w3schools.com\/collections\/course-catalog\/products\/front-end-course \"Paid Front End Development Course\")   [Bootcamp](https:\/\/www.w3schools.com\/bootcamp\/web-development.php \"Front End Development Bootcamp\")\n\n[Web Dev.](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/modern-web-development-certification \"Web Development Certification\")   [Certificate](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/modern-web-development-certification \"Web Development Certification\")   [Course](https:\/\/campus.w3schools.com\/collections\/course-best-sellers\/products\/learn-modern-web-development \"Paid Web Development Course\")   [Bootcamp](https:\/\/www.w3schools.com\/bootcamp\/web-development.php \"Web Development Bootcamp\")\n\n[Web App](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/web-application-development-certificates \"Web Application Development Certification\")   [Certificate](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/web-application-development-certificates \"Web Application Development Certification\")   [Course](https:\/\/campus.w3schools.com\/collections\/course-best-sellers\/products\/web-application-development-course \"Paid Web Application Course\")\n\n[Web Design](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/web-design-certification \"Web Design Certification Exam\")   [Certificate](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/web-design-certification \"Web Design Certification Exam\")   [Course](https:\/\/campus.w3schools.com\/collections\/course-best-sellers\/products\/learn-web-design \"Paid Web Design Course\")\n\n### Programs\n[Full Access Best Value\\!](https:\/\/campus.w3schools.com\/collections\/course-catalog\/products\/w3schools-full-access-course \"Paid Full Access Course\")\n\n[Front End](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/front-end-certificate \"Front End Certification Exam\")   [Certificate](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/front-end-certificate \"Front End Certification Exam\")   [Course](https:\/\/campus.w3schools.com\/collections\/course-catalog\/products\/front-end-course \"Paid Front End Development Course\")   [Bootcamp](https:\/\/www.w3schools.com\/bootcamp\/web-development.php \"Front End Development Bootcamp\")\n\n[Web Dev.](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/modern-web-development-certification \"Web Development Certification\")   [Certificate](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/modern-web-development-certification \"Web Development Certification\")   [Course](https:\/\/campus.w3schools.com\/collections\/course-best-sellers\/products\/learn-modern-web-development \"Paid Web Development Course\")   [Bootcamp](https:\/\/www.w3schools.com\/bootcamp\/web-development.php \"Web Development Bootcamp\")\n\n[Web App](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/web-application-development-certificates \"Web Application Development Certification\")   [Certificate](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/web-application-development-certificates \"Web Application Development Certification\")   [Course](https:\/\/campus.w3schools.com\/collections\/course-best-sellers\/products\/web-application-development-course \"Paid Web Application Course\")\n\n[Web Design](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/web-design-certification \"Web Design Certification Exam\")   [Certificate](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/web-design-certification \"Web Design Certification Exam\")   [Course](https:\/\/campus.w3schools.com\/collections\/course-best-sellers\/products\/learn-web-design \"Paid Web Design Course\")\n\n### Backend\n[Python](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/python-certificate \"Python Certification Exam\")   [Certificate](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/python-certificate \"Python Certification Exam\")   [Course](https:\/\/campus.w3schools.com\/collections\/course-catalog\/products\/python-course \"Paid Python Course\")   [Bootcamp](https:\/\/www.w3schools.com\/bootcamp\/python.php \"Python Bootcamp\")\n\n[SQL](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/sql-certificate \"SQL Certification Exam\")   [Certificate](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/sql-certificate \"SQL Certification Exam\")   [Course](https:\/\/campus.w3schools.com\/collections\/course-catalog\/products\/sql-course \"SQL Python Course\")   [Bootcamp](https:\/\/www.w3schools.com\/bootcamp\/sql.php \"SQL Bootcamp\")\n\n[MySQL](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/mysql-certificate \"MySQL Certification Exam\")   [Certificate](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/mysql-certificate \"MySQL Certification Exam\")\n\n[PHP](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/php-certificate \"PHP Certification Exam\")   [Certificate](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/php-certificate \"PHP Certification Exam\")   [Course](https:\/\/campus.w3schools.com\/collections\/course-catalog\/products\/php-course \"Paid PHP Course\")\n\n[Java](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/java-certificate \"Java Certification Exam\")   [Certificate](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/java-certificate \"Java Certification Exam\")   [Course](https:\/\/campus.w3schools.com\/collections\/course-catalog\/products\/java-course \"Paid Java Course\")\n\n[C](https:\/\/campus.w3schools.com\/products\/c-certification-exam \"C Certification Exam\")   [Certificate](https:\/\/campus.w3schools.com\/products\/c-certification-exam \"C Certification Exam\")\n\n[C++](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/c-certificate \"C++ Certification Exam\")   [Certificate](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/c-certificate \"C++ Certification Exam\")   [Course](https:\/\/campus.w3schools.com\/collections\/course-catalog\/products\/c-course-1 \"Paid C++ Course\")\n\n[C\\#](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/c-certificate-1 \"C# Certification Exam\")   [Certificate](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/c-certificate-1 \"C# Certification Exam\")   [Course](https:\/\/campus.w3schools.com\/collections\/course-catalog\/products\/c-course \"Paid C# Course\")\n\n[R](https:\/\/campus.w3schools.com\/collections\/course-catalog\/products\/r-course \"Paid R Course\")   [Course](https:\/\/campus.w3schools.com\/collections\/course-catalog\/products\/r-course \"Paid R Course\")\n\n[Django](https:\/\/campus.w3schools.com\/products\/django-certification-exam \"Django Certification Exam\")   [Certificate](https:\/\/campus.w3schools.com\/products\/django-certification-exam \"Django Certification Exam\")\n\n[NodeJS](https:\/\/campus.w3schools.com\/products\/nodejs-certificate \"NodeJS Certification Exam\")   [Certificate](https:\/\/campus.w3schools.com\/products\/nodejs-certificate \"NodeJS Certification Exam\")   [Bootcamp](https:\/\/www.w3schools.com\/bootcamp\/nodejs.php \"NodeJS Bootcamp\")\n\n[TypeScript](https:\/\/campus.w3schools.com\/collections\/certifications\/products\/typescript-certificate \"TypeScript Certification Exam\")   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Tutorial\") [R](https:\/\/www.w3schools.com\/r\/default.asp \"R Tutorial\") [GO](https:\/\/www.w3schools.com\/go\/index.php \"Go Tutorial\") [KOTLIN](https:\/\/www.w3schools.com\/kotlin\/index.php \"Kotlin Tutorial\") [SWIFT](https:\/\/www.w3schools.com\/swift\/default.asp \"Swift Tutorial\") [SASS](https:\/\/www.w3schools.com\/sass\/default.asp \"Sass Tutorial\") [VUE](https:\/\/www.w3schools.com\/vue\/index.php \"Vue.js Tutorial\") [GEN AI](https:\/\/www.w3schools.com\/gen_ai\/index.php \"Gen A.I. Tutorial\") [SCIPY](https:\/\/www.w3schools.com\/python\/scipy\/index.php \"SciPy Tutorial\") [AWS](https:\/\/www.w3schools.com\/aws\/index.php \"AWS Tutorial\") [CYBERSECURITY](https:\/\/www.w3schools.com\/cybersecurity\/index.php \"Cyber security Tutorial\") [DATA SCIENCE](https:\/\/www.w3schools.com\/datascience\/default.asp \"Data science Tutorial\") [INTRO TO PROGRAMMING](https:\/\/www.w3schools.com\/programming\/index.php \"Introduction to Programming\") [HTML & CSS](https:\/\/www.w3schools.com\/htmlcss\/default.asp \"Introduction to HTML & CSS\") [BASH](https:\/\/www.w3schools.com\/bash\/index.php \"Bash Tutorial\") [RUST](https:\/\/www.w3schools.com\/rust\/index.php \"Rust Tutorial\") [TOOLS](https:\/\/www.w3schools.com\/tools\/index.php \"Web Developer Tools\")\n\n## DSA Tutorial\n[DSA HOME](https:\/\/www.w3schools.com\/dsa\/index.php) [DSA Intro](https:\/\/www.w3schools.com\/dsa\/dsa_intro.php) [DSA Simple Algorithm](https:\/\/www.w3schools.com\/dsa\/dsa_algo_simple.php)\n\n## Arrays\n[DSA Arrays](https:\/\/www.w3schools.com\/dsa\/dsa_data_arrays.php) [DSA Bubble Sort](https:\/\/www.w3schools.com\/dsa\/dsa_algo_bubblesort.php) [DSA Selection Sort](https:\/\/www.w3schools.com\/dsa\/dsa_algo_selectionsort.php) [DSA Insertion Sort](https:\/\/www.w3schools.com\/dsa\/dsa_algo_insertionsort.php) [DSA Quick Sort](https:\/\/www.w3schools.com\/dsa\/dsa_algo_quicksort.php) [DSA Counting Sort](https:\/\/www.w3schools.com\/dsa\/dsa_algo_countingsort.php) [DSA Radix Sort](https:\/\/www.w3schools.com\/dsa\/dsa_algo_radixsort.php) [DSA Merge Sort](https:\/\/www.w3schools.com\/dsa\/dsa_algo_mergesort.php) [DSA Linear Search](https:\/\/www.w3schools.com\/dsa\/dsa_algo_linearsearch.php) [DSA Binary Search](https:\/\/www.w3schools.com\/dsa\/dsa_algo_binarysearch.php)\n\n## Linked Lists\n[DSA Linked Lists](https:\/\/www.w3schools.com\/dsa\/dsa_theory_linkedlists.php) [DSA Linked Lists in Memory](https:\/\/www.w3schools.com\/dsa\/dsa_theory_linkedlists_memory.php) [DSA Linked Lists Types](https:\/\/www.w3schools.com\/dsa\/dsa_data_linkedlists_types.php) [Linked Lists Operations](https:\/\/www.w3schools.com\/dsa\/dsa_algo_linkedlists_operations.php)\n\n## Stacks & Queues\n[DSA Stacks](https:\/\/www.w3schools.com\/dsa\/dsa_data_stacks.php) [DSA Queues](https:\/\/www.w3schools.com\/dsa\/dsa_data_queues.php)\n\n## Hash Tables\n[DSA Hash Tables](https:\/\/www.w3schools.com\/dsa\/dsa_theory_hashtables.php) [DSA Hash Sets](https:\/\/www.w3schools.com\/dsa\/dsa_data_hashsets.php) [DSA Hash Maps](https:\/\/www.w3schools.com\/dsa\/dsa_data_hashmaps.php)\n\n## Trees\n[DSA Trees](https:\/\/www.w3schools.com\/dsa\/dsa_theory_trees.php) [DSA Binary Trees](https:\/\/www.w3schools.com\/dsa\/dsa_data_binarytrees.php) [DSA Pre-order Traversal](https:\/\/www.w3schools.com\/dsa\/dsa_algo_binarytrees_preorder.php) [DSA In-order Traversal](https:\/\/www.w3schools.com\/dsa\/dsa_algo_binarytrees_inorder.php) [DSA Post-order Traversal](https:\/\/www.w3schools.com\/dsa\/dsa_algo_binarytrees_postorder.php) [DSA Array Implementation](https:\/\/www.w3schools.com\/dsa\/dsa_data_binarytrees_arrayImpl.php) [DSA Binary Search Trees](https:\/\/www.w3schools.com\/dsa\/dsa_data_binarysearchtrees.php) [DSA AVL Trees](https:\/\/www.w3schools.com\/dsa\/dsa_data_avltrees.php)\n\n## Graphs\n[DSA Graphs](https:\/\/www.w3schools.com\/dsa\/dsa_theory_graphs.php) [Graphs Implementation](https:\/\/www.w3schools.com\/dsa\/dsa_data_graphs_implementation.php) [DSA Graphs Traversal](https:\/\/www.w3schools.com\/dsa\/dsa_algo_graphs_traversal.php) [DSA Cycle Detection](https:\/\/www.w3schools.com\/dsa\/dsa_algo_graphs_cycledetection.php)\n\n## Shortest Path\n[DSA Shortest Path](https:\/\/www.w3schools.com\/dsa\/dsa_theory_graphs_shortestpath.php) [DSA Dijkstra's](https:\/\/www.w3schools.com\/dsa\/dsa_algo_graphs_dijkstra.php) [DSA Bellman-Ford](https:\/\/www.w3schools.com\/dsa\/dsa_algo_graphs_bellmanford.php)\n\n## Minimum Spanning Tree\n[Minimum Spanning Tree](https:\/\/www.w3schools.com\/dsa\/dsa_theory_mst_minspantree.php) [DSA Prim's](https:\/\/www.w3schools.com\/dsa\/dsa_algo_mst_prim.php) [DSA Kruskal's](https:\/\/www.w3schools.com\/dsa\/dsa_algo_mst_kruskal.php)\n\n## Maximum Flow\n[DSA Maximum Flow](https:\/\/www.w3schools.com\/dsa\/dsa_theory_graphs_maxflow.php) [DSA Ford-Fulkerson](https:\/\/www.w3schools.com\/dsa\/dsa_algo_graphs_fordfulkerson.php) [DSA Edmonds-Karp](https:\/\/www.w3schools.com\/dsa\/dsa_algo_graphs_edmondskarp.php)\n\n## Time Complexity\n[Introduction](https:\/\/www.w3schools.com\/dsa\/dsa_timecomplexity_theory.php) [Bubble Sort](https:\/\/www.w3schools.com\/dsa\/dsa_timecomplexity_bblsort.php) [Selection Sort](https:\/\/www.w3schools.com\/dsa\/dsa_timecomplexity_selsort.php) [Insertion Sort](https:\/\/www.w3schools.com\/dsa\/dsa_timecomplexity_insertionsort.php) [Quick Sort](https:\/\/www.w3schools.com\/dsa\/dsa_timecomplexity_quicksort.php) [Counting Sort](https:\/\/www.w3schools.com\/dsa\/dsa_timecomplexity_countsort.php) [Radix Sort](https:\/\/www.w3schools.com\/dsa\/dsa_timecomplexity_radixsort.php) [Merge Sort](https:\/\/www.w3schools.com\/dsa\/dsa_timecomplexity_mergesort.php) [Linear Search](https:\/\/www.w3schools.com\/dsa\/dsa_timecomplexity_linearsearch.php) [Binary Search](https:\/\/www.w3schools.com\/dsa\/dsa_timecomplexity_binarysearch.php)\n\n## DSA Reference\n[DSA Euclidean Algorithm](https:\/\/www.w3schools.com\/dsa\/dsa_ref_euclidean_algorithm.php) [DSA Huffman Coding](https:\/\/www.w3schools.com\/dsa\/dsa_ref_huffman_coding.php) [DSA The Traveling Salesman](https:\/\/www.w3schools.com\/dsa\/dsa_ref_traveling_salesman.php) [DSA 0\/1 Knapsack](https:\/\/www.w3schools.com\/dsa\/dsa_ref_knapsack.php) [DSA Memoization](https:\/\/www.w3schools.com\/dsa\/dsa_ref_memoization.php) [DSA Tabulation](https:\/\/www.w3schools.com\/dsa\/dsa_ref_tabulation.php) [DSA Dynamic Programming](https:\/\/www.w3schools.com\/dsa\/dsa_ref_dynamic_programming.php) [DSA Greedy Algorithms](https:\/\/www.w3schools.com\/dsa\/dsa_ref_greedy.php)\n\n## DSA Examples\n[DSA Examples](https:\/\/www.w3schools.com\/dsa\/dsa_examples.php) [DSA Exercises](https:\/\/www.w3schools.com\/dsa\/dsa_exercises.php) [DSA Quiz](https:\/\/www.w3schools.com\/dsa\/dsa_quiz.php) [DSA Syllabus](https:\/\/www.w3schools.com\/dsa\/dsa_syllabus.php) [DSA Study Plan](https:\/\/www.w3schools.com\/dsa\/dsa_study_plan.php) [DSA Certificate](https:\/\/www.w3schools.com\/dsa\/dsa_exam.php)\n\n# DSA Queues\n[❮ Previous](https:\/\/www.w3schools.com\/dsa\/dsa_data_stacks.php) [Next ❯](https:\/\/www.w3schools.com\/dsa\/dsa_theory_hashtables.php)\n***\n## Queues\nA queue is a data structure that can hold many elements.\n\n![Out sign](https:\/\/www.w3schools.com\/dsa\/img_queuessign_out.svg)\n\n3\n\n2\n\n1\n\n4\n\n![In sign](https:\/\/www.w3schools.com\/dsa\/img_queuessign_in.svg)\n\nResult:\n\nenqueue()\n\ndequeue()\n\npeek()\n\nisEmpty()\n\nsize()\n\nThink of a queue as people standing in line in a supermarket.\n\nThe first person to stand in line is also the first who can pay and leave the supermarket. This way of organizing elements is called FIFO: First In First Out.\n\nBasic operations we can do on a queue are:\n\n- **Enqueue:** Adds a new element to the queue.\n- **Dequeue:** Removes and returns the first (front) element from the queue.\n- **Peek:** Returns the first element in the queue.\n- **isEmpty:** Checks if the queue is empty.\n- **Size:** Finds the number of elements in the queue.\n\nExperiment with these basic operations in the queue animation above.\n\nQueues can be implemented by using arrays or linked lists.\n\nQueues can be used to implement job scheduling for an office printer, order processing for e-tickets, or to create algorithms for breadth-first search in graphs.\n\nQueues are often mentioned together with Stacks, which is a similar data structure described on the [previous page](https:\/\/www.w3schools.com\/dsa\/dsa_data_stacks.php).\n***\n[REMOVE ADS](https:\/\/order.w3schools.com\/plans)\n***\n## Queue Implementation using Arrays\nTo better understand the benefits with using arrays or linked lists to implement queues, you should check out [this page](https:\/\/www.w3schools.com\/dsa\/dsa_theory_linkedlists_memory.php) that explains how arrays and linked lists are stored in memory.\n\nThis is how it looks like when we use an array as a queue:\n\n\\[\n\n3,\n\n2,\n\n2,\n\n2\n\n\\]\n\nResult:\n\nenqueue()\n\ndequeue()\n\npeek()\n\nisEmpty()\n\nsize()\n\nReasons to implement queues using arrays:\n\n- **Memory Efficient:** Array elements do not hold the next elements address like linked list nodes do.\n- **Easier to implement and understand:** Using arrays to implement queues require less code than using linked lists, and for this reason it is typically easier to understand as well.\n\nReasons for **not** using arrays to implement queues:\n\n- **Fixed size:** An array occupies a fixed part of the memory. This means that it could take up more memory than needed, or if the array fills up, it cannot hold more elements. And resizing an array can be costly.\n- **Shifting cost:** Dequeue causes the first element in a queue to be removed, and the other elements must be shifted to take the removed elements' place. This is inefficient and can cause problems, especially if the queue is long.\n- **Alternatives:** Some programming languages have built-in data structures optimized for queue operations that are better than using arrays.\n\n**Note:** When using arrays in Python for this tutorial, we are really using the Python 'list' data type, but for the scope of this tutorial the 'list' data type can be used in the same way as an array. Learn more about Python lists [here](https:\/\/www.w3schools.com\/python\/python_lists.asp).\n\nSince Python lists has good support for functionality needed to implement queues, we start with creating a queue and do queue operations with just a few lines:\n\n### Example\nPython:\n```\nqueue = []\n\n# Enqueue\nqueue.append('A')\nqueue.append('B')\nqueue.append('C')\nprint(\"Queue: \", queue)\n\n# Dequeue\nelement = queue.pop(0)\nprint(\"Dequeue: \", element)\n\n# Peek\nfrontElement = queue[0]\nprint(\"Peek: \", frontElement)\n\n# isEmpty\nisEmpty = not bool(queue)\nprint(\"isEmpty: \", isEmpty)\n\n# Size\nprint(\"Size: \", len(queue))\n```\n[Try it Yourself »](https:\/\/www.w3schools.com\/dsa\/trydsa.php?filename=demo_queues_array1)\n\nBut to explicitly create a data structure for queues, with basic operations, we should create a queue class instead. This way of creating queues in Python is also more similar to how queues can be created in other programming languages like C and Java.\n\n### Example\nPython:\n```\nclass Queue:\n    def __init__(self):\n        self.queue = []\n    \n    def enqueue(self, element):\n        self.queue.append(element)\n    \n    def dequeue(self):\n        if self.isEmpty():\n            return \"Queue is empty\"\n        return self.queue.pop(0)\n    \n    def peek(self):\n        if self.isEmpty():\n            return \"Queue is empty\"\n        return self.queue[0]\n    \n    def isEmpty(self):\n        return len(self.queue) == 0\n    \n    def size(self):\n        return len(self.queue)\n\n# Create a queue\nmyQueue = Queue()\n\nmyQueue.enqueue('A')\nmyQueue.enqueue('B')\nmyQueue.enqueue('C')\nprint(\"Queue: \", myQueue.queue)\n\nprint(\"Dequeue: \", myQueue.dequeue())\n\nprint(\"Peek: \", myQueue.peek())\n\nprint(\"isEmpty: \", myQueue.isEmpty())\n\nprint(\"Size: \", myQueue.size())\n```\n[Try it Yourself »](https:\/\/www.w3schools.com\/dsa\/trydsa.php?filename=demo_queues_array2)\n***\n## Queue Implementation using Linked Lists\nReasons for using linked lists to implement queues:\n\n- **Dynamic size:** The queue can grow and shrink dynamically, unlike with arrays.\n- **No shifting:** The front element of the queue can be removed (dequeue) without having to shift other elements in the memory.\n\nReasons for **not** using linked lists to implement queues:\n\n- **Extra memory:** Each queue element must contain the address to the next element (the next linked list node).\n- **Readability:** The code might be harder to read and write for some because it is longer and more complex.\n\nThis is how a queue can be implemented using a linked list.\n\n### Example\nPython:\n```\nclass Node:\n    def __init__(self, data):\n        self.data = data\n        self.next = None\n\nclass Queue:\n    def __init__(self):\n        self.front = None\n        self.rear = None\n        self.length = 0\n    \n    def enqueue(self, element):\n        new_node = Node(element)\n        if self.rear is None:\n            self.front = self.rear = new_node\n            self.length += 1\n            return\n        self.rear.next = new_node\n        self.rear = new_node\n        self.length += 1\n    \n    def dequeue(self):\n        if self.isEmpty():\n            return \"Queue is empty\"\n        temp = self.front\n        self.front = temp.next\n        self.length -= 1\n        if self.front is None:\n            self.rear = None\n        return temp.data\n    \n    def peek(self):\n        if self.isEmpty():\n            return \"Queue is empty\"\n        return self.front.data\n    \n    def isEmpty(self):\n        return self.length == 0\n    \n    def size(self):\n        return self.length\n\n    def printQueue(self):\n        temp = self.front\n        while temp:\n            print(temp.data, end=\" \")\n            temp = temp.next\n        print()\n\n# Create a queue\nmyQueue = Queue()\n\nmyQueue.enqueue('A')\nmyQueue.enqueue('B')\nmyQueue.enqueue('C')\nprint(\"Queue: \", end=\"\")\nmyQueue.printQueue()\n\nprint(\"Dequeue: \", myQueue.dequeue())\n\nprint(\"Peek: \", myQueue.peek())\n\nprint(\"isEmpty: \", myQueue.isEmpty())\n\nprint(\"Size: \", myQueue.size())\n```\n[Try it Yourself »](https:\/\/www.w3schools.com\/dsa\/trydsa.php?filename=demo_queues_linkedlist)\n***\n## DSA Exercises\n***\n[❮ Previous](https:\/\/www.w3schools.com\/dsa\/dsa_data_stacks.php) [Next ❯](https:\/\/www.w3schools.com\/dsa\/dsa_theory_hashtables.php)\n\n[★ +1](https:\/\/profile.w3schools.com\/log-in?redirect_url=https%3A%2F%2Fwww.w3schools.com%2Fdsa%2Fdsa_data_queues.php \"Your W3Schools Profile\")\n\n[Sign in to track progress](https:\/\/profile.w3schools.com\/log-in?redirect_url=https%3A%2F%2Fpathfinder.w3schools.com&origin=https%3A%2F%2Fwww.w3schools.com%2Fdsa%2Fdsa_data_queues.php \"Sign in to track your progress\")\n\n[![Get Certified 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[W3Schools is Powered by W3.CSS](https:\/\/www.w3schools.com\/w3css\/default.asp).","attrs_readable_markdown":"## DSA Queues\n***\n## Queues\nA queue is a data structure that can hold many elements.\n\n![Out sign](https:\/\/www.w3schools.com\/dsa\/img_queuessign_out.svg)\n\n3\n\n2\n\n1\n\n4\n\n![In sign](https:\/\/www.w3schools.com\/dsa\/img_queuessign_in.svg)\n\nResult:\n\nThink of a queue as people standing in line in a supermarket.\n\nThe first person to stand in line is also the first who can pay and leave the supermarket. This way of organizing elements is called FIFO: First In First Out.\n\nBasic operations we can do on a queue are:\n\n- **Enqueue:** Adds a new element to the queue.\n- **Dequeue:** Removes and returns the first (front) element from the queue.\n- **Peek:** Returns the first element in the queue.\n- **isEmpty:** Checks if the queue is empty.\n- **Size:** Finds the number of elements in the queue.\n\nExperiment with these basic operations in the queue animation above.\n\nQueues can be implemented by using arrays or linked lists.\n\nQueues can be used to implement job scheduling for an office printer, order processing for e-tickets, or to create algorithms for breadth-first search in graphs.\n\nQueues are often mentioned together with Stacks, which is a similar data structure described on the [previous page](https:\/\/www.w3schools.com\/dsa\/dsa_data_stacks.php).\n***\n***\n## Queue Implementation using Arrays\nTo better understand the benefits with using arrays or linked lists to implement queues, you should check out [this page](https:\/\/www.w3schools.com\/dsa\/dsa_theory_linkedlists_memory.php) that explains how arrays and linked lists are stored in memory.\n\nThis is how it looks like when we use an array as a queue:\n\n\\[\n\n3,\n\n2,\n\n2,\n\n2\n\n\\]\n\nResult:\n\nReasons to implement queues using arrays:\n\n- **Memory Efficient:** Array elements do not hold the next elements address like linked list nodes do.\n- **Easier to implement and understand:** Using arrays to implement queues require less code than using linked lists, and for this reason it is typically easier to understand as well.\n\nReasons for **not** using arrays to implement queues:\n\n- **Fixed size:** An array occupies a fixed part of the memory. This means that it could take up more memory than needed, or if the array fills up, it cannot hold more elements. And resizing an array can be costly.\n- **Shifting cost:** Dequeue causes the first element in a queue to be removed, and the other elements must be shifted to take the removed elements' place. This is inefficient and can cause problems, especially if the queue is long.\n- **Alternatives:** Some programming languages have built-in data structures optimized for queue operations that are better than using arrays.\n\n**Note:** When using arrays in Python for this tutorial, we are really using the Python 'list' data type, but for the scope of this tutorial the 'list' data type can be used in the same way as an array. Learn more about Python lists [here](https:\/\/www.w3schools.com\/python\/python_lists.asp).\n\nSince Python lists has good support for functionality needed to implement queues, we start with creating a queue and do queue operations with just a few lines:\n\n### Example\nPython:\n```\nqueue = []\n\n# Enqueue\nqueue.append('A')\nqueue.append('B')\nqueue.append('C')\nprint(\"Queue: \", queue)\n\n# Dequeue\nelement = queue.pop(0)\nprint(\"Dequeue: \", element)\n\n# Peek\nfrontElement = queue[0]\nprint(\"Peek: \", frontElement)\n\n# isEmpty\nisEmpty = not bool(queue)\nprint(\"isEmpty: \", isEmpty)\n\n# Size\nprint(\"Size: \", len(queue))\n```\n[Try it Yourself »](https:\/\/www.w3schools.com\/dsa\/trydsa.php?filename=demo_queues_array1)\n\nBut to explicitly create a data structure for queues, with basic operations, we should create a queue class instead. This way of creating queues in Python is also more similar to how queues can be created in other programming languages like C and Java.\n\n### Example\nPython:\n```\nclass Queue:\n    def __init__(self):\n        self.queue = []\n    \n    def enqueue(self, element):\n        self.queue.append(element)\n    \n    def dequeue(self):\n        if self.isEmpty():\n            return \"Queue is empty\"\n        return self.queue.pop(0)\n    \n    def peek(self):\n        if self.isEmpty():\n            return \"Queue is empty\"\n        return self.queue[0]\n    \n    def isEmpty(self):\n        return len(self.queue) == 0\n    \n    def size(self):\n        return len(self.queue)\n\n# Create a queue\nmyQueue = Queue()\n\nmyQueue.enqueue('A')\nmyQueue.enqueue('B')\nmyQueue.enqueue('C')\nprint(\"Queue: \", myQueue.queue)\n\nprint(\"Dequeue: \", myQueue.dequeue())\n\nprint(\"Peek: \", myQueue.peek())\n\nprint(\"isEmpty: \", myQueue.isEmpty())\n\nprint(\"Size: \", myQueue.size())\n```\n[Try it Yourself »](https:\/\/www.w3schools.com\/dsa\/trydsa.php?filename=demo_queues_array2)\n***\n## Queue Implementation using Linked Lists\nReasons for using linked lists to implement queues:\n\n- **Dynamic size:** The queue can grow and shrink dynamically, unlike with arrays.\n- **No shifting:** The front element of the queue can be removed (dequeue) without having to shift other elements in the memory.\n\nReasons for **not** using linked lists to implement queues:\n\n- **Extra memory:** Each queue element must contain the address to the next element (the next linked list node).\n- **Readability:** The code might be harder to read and write for some because it is longer and more complex.\n\nThis is how a queue can be implemented using a linked list.\n\n### Example\nPython:\n```\nclass Node:\n    def __init__(self, data):\n        self.data = data\n        self.next = None\n\nclass Queue:\n    def __init__(self):\n        self.front = None\n        self.rear = None\n        self.length = 0\n    \n    def enqueue(self, element):\n        new_node = Node(element)\n        if self.rear is None:\n            self.front = self.rear = new_node\n            self.length += 1\n            return\n        self.rear.next = new_node\n        self.rear = new_node\n        self.length += 1\n    \n    def dequeue(self):\n        if self.isEmpty():\n            return \"Queue is empty\"\n        temp = self.front\n        self.front = temp.next\n        self.length -= 1\n        if self.front is None:\n            self.rear = None\n        return temp.data\n    \n    def peek(self):\n        if self.isEmpty():\n            return \"Queue is empty\"\n        return self.front.data\n    \n    def isEmpty(self):\n        return self.length == 0\n    \n    def size(self):\n        return self.length\n\n    def printQueue(self):\n        temp = self.front\n        while temp:\n            print(temp.data, end=\" \")\n            temp = temp.next\n        print()\n\n# Create a queue\nmyQueue = Queue()\n\nmyQueue.enqueue('A')\nmyQueue.enqueue('B')\nmyQueue.enqueue('C')\nprint(\"Queue: \", end=\"\")\nmyQueue.printQueue()\n\nprint(\"Dequeue: \", myQueue.dequeue())\n\nprint(\"Peek: \", myQueue.peek())\n\nprint(\"isEmpty: \", myQueue.isEmpty())\n\nprint(\"Size: \", myQueue.size())\n```\n[Try it Yourself »](https:\/\/www.w3schools.com\/dsa\/trydsa.php?filename=demo_queues_linkedlist)\n***\n***\n\n[![Get Certified Offer](https:\/\/www.w3schools.com\/images\/img_2026easter_fullaccess_300.webp)](https:\/\/campus.w3schools.com\/products\/w3schools-full-access-course)","meta_canonical":null}

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Boilerpipe Text	DSA Queues Queues A queue is a data structure that can hold many elements. 3 2 1 4 Result: Think of a queue as people standing in line in a supermarket. The first person to stand in line is also the first who can pay and leave the supermarket. This way of organizing elements is called FIFO: First In First Out. Basic operations we can do on a queue are: Enqueue: Adds a new element to the queue. Dequeue: Removes and returns the first (front) element from the queue. Peek: Returns the first element in the queue. isEmpty: Checks if the queue is empty. Size: Finds the number of elements in the queue. Experiment with these basic operations in the queue animation above. Queues can be implemented by using arrays or linked lists. Queues can be used to implement job scheduling for an office printer, order processing for e-tickets, or to create algorithms for breadth-first search in graphs. Queues are often mentioned together with Stacks, which is a similar data structure described on the previous page . Queue Implementation using Arrays To better understand the benefits with using arrays or linked lists to implement queues, you should check out this page that explains how arrays and linked lists are stored in memory. This is how it looks like when we use an array as a queue: [ 3 , 2 , 2 , 2 ] Result: Reasons to implement queues using arrays: Memory Efficient: Array elements do not hold the next elements address like linked list nodes do. Easier to implement and understand: Using arrays to implement queues require less code than using linked lists, and for this reason it is typically easier to understand as well. Reasons for not using arrays to implement queues: Fixed size: An array occupies a fixed part of the memory. This means that it could take up more memory than needed, or if the array fills up, it cannot hold more elements. And resizing an array can be costly. Shifting cost: Dequeue causes the first element in a queue to be removed, and the other elements must be shifted to take the removed elements' place. This is inefficient and can cause problems, especially if the queue is long. Alternatives: Some programming languages have built-in data structures optimized for queue operations that are better than using arrays. Note: When using arrays in Python for this tutorial, we are really using the Python 'list' data type, but for the scope of this tutorial the 'list' data type can be used in the same way as an array. Learn more about Python lists here . Since Python lists has good support for functionality needed to implement queues, we start with creating a queue and do queue operations with just a few lines: Example Python: queue = [ ] # Enqueue queue . append ( 'A' ) queue . append ( 'B' ) queue . append ( 'C' ) print ( "Queue: " , queue ) # Dequeue element = queue . pop ( 0 ) print ( "Dequeue: " , element ) # Peek frontElement = queue [ 0 ] print ( "Peek: " , frontElement ) # isEmpty isEmpty = not bool ( queue ) print ( "isEmpty: " , isEmpty ) # Size print ( "Size: " , len ( queue ) ) Try it Yourself » But to explicitly create a data structure for queues, with basic operations, we should create a queue class instead. This way of creating queues in Python is also more similar to how queues can be created in other programming languages like C and Java. Example Python: class Queue : def __init__ ( self ) : self . queue = [ ] def enqueue ( self , element ) : self . queue . append ( element ) def dequeue ( self ) : if self . isEmpty ( ) : return "Queue is empty" return self . queue . pop ( 0 ) def peek ( self ) : if self . isEmpty ( ) : return "Queue is empty" return self . queue [ 0 ] def isEmpty ( self ) : return len ( self . queue ) == 0 def size ( self ) : return len ( self . queue ) # Create a queue myQueue = Queue ( ) myQueue . enqueue ( 'A' ) myQueue . enqueue ( 'B' ) myQueue . enqueue ( 'C' ) print ( "Queue: " , myQueue . queue ) print ( "Dequeue: " , myQueue . dequeue ( ) ) print ( "Peek: " , myQueue . peek ( ) ) print ( "isEmpty: " , myQueue . isEmpty ( ) ) print ( "Size: " , myQueue . size ( ) ) Try it Yourself » Queue Implementation using Linked Lists Reasons for using linked lists to implement queues: Dynamic size: The queue can grow and shrink dynamically, unlike with arrays. No shifting: The front element of the queue can be removed (dequeue) without having to shift other elements in the memory. Reasons for not using linked lists to implement queues: Extra memory: Each queue element must contain the address to the next element (the next linked list node). Readability: The code might be harder to read and write for some because it is longer and more complex. This is how a queue can be implemented using a linked list. Example Python: class Node : def __init__ ( self , data ) : self . data = data self . next = None class Queue : def __init__ ( self ) : self . front = None self . rear = None self . length = 0 def enqueue ( self , element ) : new_node = Node ( element ) if self . rear is None : self . front = self . rear = new_node self . length += 1 return self . rear . next = new_node self . rear = new_node self . length += 1 def dequeue ( self ) : if self . isEmpty ( ) : return "Queue is empty" temp = self . front self . front = temp . next self . length -= 1 if self . front is None : self . rear = None return temp . data def peek ( self ) : if self . isEmpty ( ) : return "Queue is empty" return self . front . data def isEmpty ( self ) : return self . length == 0 def size ( self ) : return self . length def printQueue ( self ) : temp = self . front while temp : print ( temp . data , end = " " ) temp = temp . next print ( ) # Create a queue myQueue = Queue ( ) myQueue . enqueue ( 'A' ) myQueue . enqueue ( 'B' ) myQueue . enqueue ( 'C' ) print ( "Queue: " , end = "" ) myQueue . printQueue ( ) print ( "Dequeue: " , myQueue . dequeue ( ) ) print ( "Peek: " , myQueue . peek ( ) ) print ( "isEmpty: " , myQueue . isEmpty ( ) ) print ( "Size: " , myQueue . size ( ) ) Try it Yourself »
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"Statistics Tutorial") [Learn Excel](https://www.w3schools.com/excel/index.php "Excel Tutorial") [Tutorial](https://www.w3schools.com/excel/index.php "Excel Tutorial") [Learn Google Sheets](https://www.w3schools.com/googlesheets/index.php "Google Sheets Tutorial") [Tutorial](https://www.w3schools.com/googlesheets/index.php "Google Sheets Tutorial") ### Web Building [Create a Website](https://www.w3schools.com/spaces/index.php "Get Your Own Website With W3schools Spaces") [Create a Server NEW](https://www.w3schools.com/spaces/index.php "Get Your Own Server With W3schools Spaces") [Where To Start](https://www.w3schools.com/where_to_start.asp "Where To Start - Web Development") [Web Templates](https://www.w3schools.com/w3css/w3css_templates.asp "Free Web Templates") [Web Statistics](https://www.w3schools.com/browsers/default.asp "Web Statistics") [Web Certificates](https://campus.w3schools.com/ "Certificates") [Web Development](https://www.w3schools.com/whatis/default.asp "Web Development Roadmaps") [Introduction to Programming](https://www.w3schools.com/programming/index.php "Introduction to Programming") [Introduction to HTML & CSS](https://www.w3schools.com/htmlcss/default.asp "HTML & CSS") [Code Editor](https://www.w3schools.com/tryit/default.asp "Try it - Code Editor") [Test Your Typing Speed](https://www.w3schools.com/typingspeed/default.asp "Test Your Typing Speed") [Play a Code Game](https://www.w3schools.com/codegame/index.html "Play a Code Game") [Web Developer Tools](https://www.w3schools.com/tools/index.php "Web Developer Tools") [Cyber Security](https://www.w3schools.com/cybersecurity/index.php "Cyber Security Tutorial") [Accessibility](https://www.w3schools.com/accessibility/index.php "Accessibility Security Tutorial") [Join our Newsletter](https://campus.w3schools.com/pages/newsletter "W3Schools Newsletter") × ## References References filter input × ### HTML and CSS [HTML Tags Reference](https://www.w3schools.com/tags/default.asp "HTML Tag Reference") [CSS Reference](https://www.w3schools.com/cssref/default.asp "CSS Reference") [W3.CSS Reference](https://www.w3schools.com/w3css/w3css_references.asp "W3.CSS Reference") [Bootstrap 3 Reference](https://www.w3schools.com/bootstrap/bootstrap_ref_all_classes.asp "Bootstrap3 Reference") [Bootstrap 4 Reference](https://www.w3schools.com/bootstrap4/bootstrap_ref_all_classes.asp "Bootstrap4 Reference") [Color Names](https://www.w3schools.com/colors/colors_names.asp "Color Names") [Icons Reference](https://www.w3schools.com/icons/default.asp "Icons Reference") [SVG Reference](https://www.w3schools.com/graphics/svg_reference.asp "SVG Reference") [Canvas Reference](https://www.w3schools.com/graphics/canvas_reference.asp "Canvas Reference") [Sass Reference](https://www.w3schools.com/sass/sass_functions_string.php "SASS Reference") [UTF-8 Charset Reference](https://www.w3schools.com/charsets/default.asp "UTF-8 Reference") [UTF-8 Emojis Reference](https://www.w3schools.com/charsets/ref_emoji_smileys.asp "UTF-8 Emojis Reference") ### JavaScript [JavaScript Reference](https://www.w3schools.com/jsref/default.asp "JavaScript Reference") [HTML DOM Reference](https://www.w3schools.com/jsref/dom_obj_document.asp "HTML DOM Reference") [jQuery Reference](https://www.w3schools.com/jquery/jquery_ref_overview.asp "jQuery Reference") [Vue Reference](https://www.w3schools.com/vue/vue_ref_builtin-attributes.php "Vue Reference") [Angular Reference](https://www.w3schools.com/angular/angularjs_ref_directives.asp "Angular Reference") [JSON Reference](https://www.w3schools.com/jsref/jsref_obj_json.asp "JSON Reference") [AppML Reference](https://www.w3schools.com/appml/appml_reference.asp "AppML Reference") [W3.JS Reference](https://www.w3schools.com/e3js/w3js_references.asp "W3.JS Reference") ### Backend [Python Reference](https://www.w3schools.com/python/python_reference.asp "Python Reference") [SQL Reference](https://www.w3schools.com/sql/sql_ref_keywords.asp "SQL Reference") [MySQL Reference](https://www.w3schools.com/mysql/mysql_datatypes.asp "MySQL Reference") [PHP Reference](https://www.w3schools.com/php/php_ref_overview.asp "PHP Reference") [Java Reference](https://www.w3schools.com/java/java_ref_reference.asp "Java Reference") [C Reference](https://www.w3schools.com/c/c_ref_reference.php "C Reference") [C++ Reference](https://www.w3schools.com/cpp/cpp_ref_reference.asp "C++ Reference") [Django Reference](https://www.w3schools.com/django/django_ref_template_tags.php "Django Reference") [ASP Reference](https://www.w3schools.com/asp/asp_ref_vbscript_functions.asp "ASP Reference") [Node.js Reference](https://www.w3schools.com/nodejs/ref_modules.asp "Node.js Reference") [XML Reference](https://www.w3schools.com/xml/dom_nodetype.asp "XML Reference") × ## Exercises Excercises filter input × ### HTML and CSS [HTML](https://www.w3schools.com/html/html_exercises.asp "HTML Exercises") [Exercise](https://www.w3schools.com/html/html_exercises.asp "HTML Exercises") [Quiz](https://www.w3schools.com/html/html_quiz.asp "HTML Quizzes") [Challenge](https://www.w3schools.com/html/html_challenges.asp "HTML Code Challenges") [CSS](https://www.w3schools.com/css/css_exercises.asp "CSS Exercises") [Exercise](https://www.w3schools.com/css/css_exercises.asp "CSS Exercises") [Quiz](https://www.w3schools.com/css/css_quiz.asp "CSS Quizzes") [Challenge](https://www.w3schools.com/css/css_challenges.asp "CSS Code Challenges") [Bootstrap 3](https://www.w3schools.com/bootstrap/bootstrap_exercises.asp "Bootstrap 3 Exercises") [Exercise](https://www.w3schools.com/bootstrap/bootstrap_exercises.asp "Bootstrap 3 Exercises") [Quiz](https://www.w3schools.com/bootstrap/bootstrap_quiz.asp "Bootstrap 3 Quizzes") [Bootstrap 4](https://www.w3schools.com/bootstrap4/bootstrap_exercises.asp "Bootstrap 4 Exercises") [Exercise](https://www.w3schools.com/bootstrap4/bootstrap_exercises.asp "Bootstrap 4 Exercises") [Quiz](https://www.w3schools.com/bootstrap4/bootstrap_quiz.asp "Bootstrap 4 Quizzes") [Bootstrap 5](https://www.w3schools.com/bootstrap5/bootstrap_exercises.php "Bootstrap 5 Exercises") [Exercise](https://www.w3schools.com/bootstrap5/bootstrap_exercises.php "Bootstrap 5 Exercises") [Quiz](https://www.w3schools.com/bootstrap5/bootstrap_quiz.php "Bootstrap 5 Quizzes") ### Data Analytics [DSA](https://www.w3schools.com/dsa/dsa_exercises.php "DSA Exercises") [Exercise](https://www.w3schools.com/dsa/dsa_exercises.php "DSA Exercises") [Quiz](https://www.w3schools.com/dsa/dsa_quiz.php "DSA Quizzes") [NumPy](https://www.w3schools.com/python/numpy/numpy_exercises.asp "NumPy Exercises") [Exercise](https://www.w3schools.com/python/numpy/numpy_exercises.asp "NumPy Exercises") [Quiz](https://www.w3schools.com/python/numpy/numpy_quiz.asp "NumPy Quizzes") [Pandas](https://www.w3schools.com/python/pandas/pandas_exercises.asp "Pandas Exercises") [Exercise](https://www.w3schools.com/python/pandas/pandas_exercises.asp "Pandas Exercises") [Quiz](https://www.w3schools.com/python/pandas/pandas_quiz.asp "Pandas Quizzes") [SciPy](https://www.w3schools.com/python/scipy/scipy_exercises.php "SciPy Exercises") [Exercise](https://www.w3schools.com/python/scipy/scipy_exercises.php "SciPy Exercises") [Quiz](https://www.w3schools.com/python/scipy/scipy_quiz.php "SciPy Quizzes") [Excel](https://www.w3schools.com/excel/excel_exercises.php "Excel Exercises") [Exercise](https://www.w3schools.com/excel/excel_exercises.php "Excel Exercises") [What is an Exercise?](https://www.w3schools.com/exercises/index.php "W3Schools Exercises") [What is a Quiz?](https://www.w3schools.com/quiztest/default.asp "W3Schools Quizzes") [What is a Challenge?](https://www.w3schools.com/challenges/index.php "W3Schools Challenges") ### JavaScript [JavaScript](https://www.w3schools.com/js/js_exercises.asp "JavaScript Exercises") [Exercise](https://www.w3schools.com/js/js_exercises.asp "JavaScript Exercises") [Quiz](https://www.w3schools.com/js/js_quiz.asp "JavaScript Quizzes") [React](https://www.w3schools.com/react/react_exercises.asp "React Exercises") [Exercise](https://www.w3schools.com/react/react_exercises.asp "React Exercises") [Quiz](https://www.w3schools.com/react/react_quiz.asp "React Quizzes") [jQuery](https://www.w3schools.com/jquery/jquery_exercises.asp "jQuery Exercises") [Exercise](https://www.w3schools.com/jquery/jquery_exercises.asp "jQuery Exercises") [Quiz](https://www.w3schools.com/jquery/jquery_quiz.asp "jQuery Quizzes") [Vue](https://www.w3schools.com/vue/vue_exercises.php "Vue Exercises") [Exercise](https://www.w3schools.com/vue/vue_exercises.php "Vue Exercises") [Quiz](https://www.w3schools.com/vue/vue_quiz.php "Vue Quizzes") [Angular](https://www.w3schools.com/angular/angular_exercises.asp "Angular Exercises") [Exercise](https://www.w3schools.com/angular/angular_exercises.asp "Angular Exercises") [Quiz](https://www.w3schools.com/angular/angular_quiz.asp "Angular Quizzes") ### Backend [Python](https://www.w3schools.com/python/python_exercises.asp "Python Exercises") [Exercise](https://www.w3schools.com/python/python_exercises.asp "Python Exercises") [Quiz](https://www.w3schools.com/python/python_quiz.asp "Python Quizzes") [Challenge](https://www.w3schools.com/python/python_challenges.asp "Python Code Challenges") [SQL](https://www.w3schools.com/sql/sql_exercises.asp "SQL Exercises") [Exercise](https://www.w3schools.com/sql/sql_exercises.asp "SQL Exercises") [Quiz](https://www.w3schools.com/sql/sql_quiz.asp "SQL Quizzes") [MySQL](https://www.w3schools.com/mysql/mysql_exercises.asp "MySQL Exercises") [Exercise](https://www.w3schools.com/mysql/mysql_exercises.asp "MySQL Exercises") [Quiz](https://www.w3schools.com/mysql/mysql_quiz.asp "MySQL Quizzes") [PHP](https://www.w3schools.com/php/php_exercises.asp "PHP Exercises") [Exercise](https://www.w3schools.com/php/php_exercises.asp "PHP Exercises") [Quiz](https://www.w3schools.com/php/php_quiz.asp "PHP Quizzes") [Java](https://www.w3schools.com/java/java_exercises.asp "Java Exercises") [Exercise](https://www.w3schools.com/java/java_exercises.asp "Java Exercises") [Quiz](https://www.w3schools.com/java/java_quiz.asp "Java Quizzes") [Challenge](https://www.w3schools.com/java/java_challenges.asp "Java Code Challenges") [C](https://www.w3schools.com/c/c_exercises.php "C Exercises") [Exercise](https://www.w3schools.com/c/c_exercises.php "C Exercises") [Quiz](https://www.w3schools.com/c/c_quiz.php "C Quizzes") [Challenge](https://www.w3schools.com/c/c_challenges.php "C Challenges") [C++](https://www.w3schools.com/cpp/cpp_exercises.asp "C++ Exercises") [Exercise](https://www.w3schools.com/cpp/cpp_exercises.asp "C++ Exercises") [Quiz](https://www.w3schools.com/cpp/cpp_quiz.asp "C++ Quizzes") [Challenge](https://www.w3schools.com/cpp/cpp_challenges.asp "C++ Code Challenges") [C\#](https://www.w3schools.com/cs/cs_exercises.php "C# Exercises") [Exercise](https://www.w3schools.com/cs/cs_exercises.php "C# Exercises") [Quiz](https://www.w3schools.com/cs/cs_quiz.php "C# Quizzes") [R](https://www.w3schools.com/r/r_exercises.asp "R Exercises") [Exercise](https://www.w3schools.com/r/r_exercises.asp "R Exercises") [Quiz](https://www.w3schools.com/r/r_quiz.asp "R Quizzes") [Kotlin](https://www.w3schools.com/kotlin/kotlin_exercises.php "Kotlin Exercises") [Exercise](https://www.w3schools.com/kotlin/kotlin_exercises.php "Kotlin Exercises") [Quiz](https://www.w3schools.com/kotlin/kotlin_quiz.php "Kotlin Quizzes") [Django](https://www.w3schools.com/django/django_exercises.php "Django Exercises") [Exercise](https://www.w3schools.com/django/django_exercises.php "Django Exercises") [Quiz](https://www.w3schools.com/django/django_quiz.php "Django Quizzes") [Node.js](https://www.w3schools.com/nodejs/nodejs_exercises.asp "Node.js Exercises") [Exercise](https://www.w3schools.com/nodejs/nodejs_exercises.asp "Node.js Exercises") [Quiz](https://www.w3schools.com/nodejs/nodejs_quiz.asp "Node.js Quizzes") [PostgreSQL](https://www.w3schools.com/postgresql/postgresql_exercises.php "PostgreSQL Exercises") [Exercise](https://www.w3schools.com/postgresql/postgresql_exercises.php "PostgreSQL Exercises") [Quiz](https://www.w3schools.com/postgresql/postgresql_quiz.php "PostgreSQL Quizzes") [TypeScript](https://www.w3schools.com/typescript/typescript_exercises.php "TypeScript Exercises") [Exercise](https://www.w3schools.com/typescript/typescript_exercises.php "TypeScript Exercises") [Quiz](https://www.w3schools.com/typescript/typescript_quiz.php "TypeScript Quizzes") [Git](https://www.w3schools.com/git/git_exercises.asp "Git Exercises") [Exercise](https://www.w3schools.com/git/git_exercises.asp "Git Exercises") [Quiz](https://www.w3schools.com/git/git_quiz.asp "Git Quizzes") [Bash](https://www.w3schools.com/bash/bash_exercises.php "Bash Exercises") [Exercise](https://www.w3schools.com/bash/bash_exercises.php "Bash Exercises") [Quiz](https://www.w3schools.com/bash/bash_quiz.php "Bash Quizzes") [Go](https://www.w3schools.com/go/go_exercises.php "Go Exercises") [Exercise](https://www.w3schools.com/go/go_exercises.php "Go Exercises") [MongoDB](https://www.w3schools.com/mongodb/mongodb_exercises.php "MongoDB Exercises") [Exercise](https://www.w3schools.com/mongodb/mongodb_exercises.php "MongoDB Exercises") ### Data Analytics [DSA](https://www.w3schools.com/dsa/dsa_exercises.php "DSA Exercises") [Exercise](https://www.w3schools.com/dsa/dsa_exercises.php "DSA Exercises") [Quiz](https://www.w3schools.com/dsa/dsa_quiz.php "DSA Quizzes") [NumPy](https://www.w3schools.com/python/numpy/numpy_exercises.asp "NumPy Exercises") [Exercise](https://www.w3schools.com/python/numpy/numpy_exercises.asp "NumPy Exercises") [Quiz](https://www.w3schools.com/python/numpy/numpy_quiz.asp "NumPy Quizzes") [Pandas](https://www.w3schools.com/python/pandas/pandas_exercises.asp "Pandas Exercises") [Exercise](https://www.w3schools.com/python/pandas/pandas_exercises.asp "Pandas Exercises") [Quiz](https://www.w3schools.com/python/pandas/pandas_quiz.asp "Pandas Quizzes") [SciPy](https://www.w3schools.com/python/scipy/scipy_exercises.php "SciPy Exercises") [Exercise](https://www.w3schools.com/python/scipy/scipy_exercises.php "SciPy Exercises") [Quiz](https://www.w3schools.com/python/scipy/scipy_quiz.php "SciPy Quizzes") [Excel](https://www.w3schools.com/excel/excel_exercises.php "Excel Exercises") [Exercise](https://www.w3schools.com/excel/excel_exercises.php "Excel Exercises") [What is an Exercise?](https://www.w3schools.com/exercises/index.php "W3Schools Exercises") [What is a Quiz?](https://www.w3schools.com/quiztest/default.asp "W3Schools Quizzes") [What is a Code Challenge?](https://www.w3schools.com/challenges/index.php "W3Schools Code Challenges") × ## Certificates Filter field for certifications × ### HTML and CSS [HTML](https://campus.w3schools.com/collections/certifications/products/html-certificate "HTML Certification Exam") [Certificate](https://campus.w3schools.com/collections/certifications/products/html-certificate "HTML Certification Exam") [Course](https://campus.w3schools.com/collections/course-catalog/products/html-course "Paid HTML Course") [Bootcamp](https://www.w3schools.com/bootcamp/html-css.php "HTML Bootcamp") [CSS](https://campus.w3schools.com/collections/certifications/products/css-certificate "CSS Certification Exam") [Certificate](https://campus.w3schools.com/collections/certifications/products/css-certificate "CSS Certification Exam") [Course](https://campus.w3schools.com/collections/course-catalog/products/css-course "Paid CSS Course") [Bootcamp](https://www.w3schools.com/bootcamp/html-css.php "CSS Bootcamp") [Bootstrap 3](https://campus.w3schools.com/collections/certifications/products/bootstrap-3-certificate "Bootstrap 3 Certification Exam") [Certificate](https://campus.w3schools.com/collections/certifications/products/bootstrap-3-certificate "Bootstrap 3 Certification Exam") [Course](https://campus.w3schools.com/collections/single-courses/products/bootstrap-course "Paid Bootstrap 3 Course") [Bootstrap 4](https://campus.w3schools.com/collections/certifications/products/bootstrap-4-certificate "Bootstrap 4 Certification Exam") [Certificate](https://campus.w3schools.com/collections/certifications/products/bootstrap-4-certificate "Bootstrap 4 Certification Exam") [Course](https://campus.w3schools.com/collections/single-courses/products/bootstrap-4-course "Paid Bootstrap 4 Course") [Bootstrap 5](https://campus.w3schools.com/collections/certifications/products/bootstrap-5-certificate "Bootstrap 5 Certification Exam") [Certificate](https://campus.w3schools.com/collections/certifications/products/bootstrap-5-certificate "Bootstrap 5 Certification Exam") ### Data Analytics [DSA](https://campus.w3schools.com/products/dsa-certification-exam "DSA Certificate Exam") [Certificate](https://campus.w3schools.com/products/dsa-certification-exam "DSA Certificate Exam") [Data Analytics](https://campus.w3schools.com/products/data-analytics-program "Paid Data Analytics Course") [Course](https://campus.w3schools.com/products/data-analytics-program "Paid Data Analytics Course") [NumPy](https://campus.w3schools.com/products/numpy-certification-exam "NumPy Certification Exam") [Certificate](https://campus.w3schools.com/products/numpy-certification-exam "NumPy Certification Exam") [Course](https://campus.w3schools.com/products/numpy-course "Paid NumPy Course") [Pandas](https://campus.w3schools.com/products/pandas-certification-exam "Pandas Certification Exam") [Certificate](https://campus.w3schools.com/products/pandas-certification-exam "Pandas Certification Exam") [Course](https://campus.w3schools.com/products/pandas-course "Paid Pandas Course") [Excel](https://campus.w3schools.com/products/excel-certificate "Excel Certification Exam") [Certificate](https://campus.w3schools.com/products/excel-certificate "Excel Certification Exam") [Social Media](https://campus.w3schools.com/collections/course-best-sellers/products/social-media-marketing-course "Paid Social Media Course") [Course](https://campus.w3schools.com/collections/course-best-sellers/products/social-media-marketing-course "Paid Social Media Course") [What is a Certificate?](https://campus.w3schools.com/ "W3Schools Campus") [What is a Bootcamp?](https://www.w3schools.com/bootcamp/index.php "W3Schools Bootcamps") ### Programs [Full Access Best Value\!](https://campus.w3schools.com/collections/course-catalog/products/w3schools-full-access-course "Paid Full Access Course") [Front End](https://campus.w3schools.com/collections/certifications/products/front-end-course "Paid Front End Course") 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Linked Lists [DSA Linked Lists](https://www.w3schools.com/dsa/dsa_theory_linkedlists.php) [DSA Linked Lists in Memory](https://www.w3schools.com/dsa/dsa_theory_linkedlists_memory.php) [DSA Linked Lists Types](https://www.w3schools.com/dsa/dsa_data_linkedlists_types.php) [Linked Lists Operations](https://www.w3schools.com/dsa/dsa_algo_linkedlists_operations.php) ## Stacks & Queues [DSA Stacks](https://www.w3schools.com/dsa/dsa_data_stacks.php) [DSA Queues](https://www.w3schools.com/dsa/dsa_data_queues.php) ## Hash Tables [DSA Hash Tables](https://www.w3schools.com/dsa/dsa_theory_hashtables.php) [DSA Hash Sets](https://www.w3schools.com/dsa/dsa_data_hashsets.php) [DSA Hash Maps](https://www.w3schools.com/dsa/dsa_data_hashmaps.php) ## Trees [DSA Trees](https://www.w3schools.com/dsa/dsa_theory_trees.php) [DSA Binary Trees](https://www.w3schools.com/dsa/dsa_data_binarytrees.php) [DSA Pre-order Traversal](https://www.w3schools.com/dsa/dsa_algo_binarytrees_preorder.php) [DSA In-order Traversal](https://www.w3schools.com/dsa/dsa_algo_binarytrees_inorder.php) [DSA Post-order Traversal](https://www.w3schools.com/dsa/dsa_algo_binarytrees_postorder.php) [DSA Array Implementation](https://www.w3schools.com/dsa/dsa_data_binarytrees_arrayImpl.php) [DSA Binary Search Trees](https://www.w3schools.com/dsa/dsa_data_binarysearchtrees.php) [DSA AVL Trees](https://www.w3schools.com/dsa/dsa_data_avltrees.php) ## Graphs [DSA Graphs](https://www.w3schools.com/dsa/dsa_theory_graphs.php) [Graphs Implementation](https://www.w3schools.com/dsa/dsa_data_graphs_implementation.php) [DSA Graphs Traversal](https://www.w3schools.com/dsa/dsa_algo_graphs_traversal.php) [DSA Cycle Detection](https://www.w3schools.com/dsa/dsa_algo_graphs_cycledetection.php) ## Shortest Path [DSA Shortest Path](https://www.w3schools.com/dsa/dsa_theory_graphs_shortestpath.php) [DSA Dijkstra's](https://www.w3schools.com/dsa/dsa_algo_graphs_dijkstra.php) [DSA Bellman-Ford](https://www.w3schools.com/dsa/dsa_algo_graphs_bellmanford.php) ## Minimum Spanning Tree [Minimum Spanning Tree](https://www.w3schools.com/dsa/dsa_theory_mst_minspantree.php) [DSA Prim's](https://www.w3schools.com/dsa/dsa_algo_mst_prim.php) [DSA Kruskal's](https://www.w3schools.com/dsa/dsa_algo_mst_kruskal.php) ## Maximum Flow [DSA Maximum Flow](https://www.w3schools.com/dsa/dsa_theory_graphs_maxflow.php) [DSA Ford-Fulkerson](https://www.w3schools.com/dsa/dsa_algo_graphs_fordfulkerson.php) [DSA Edmonds-Karp](https://www.w3schools.com/dsa/dsa_algo_graphs_edmondskarp.php) ## Time Complexity [Introduction](https://www.w3schools.com/dsa/dsa_timecomplexity_theory.php) [Bubble Sort](https://www.w3schools.com/dsa/dsa_timecomplexity_bblsort.php) [Selection Sort](https://www.w3schools.com/dsa/dsa_timecomplexity_selsort.php) [Insertion Sort](https://www.w3schools.com/dsa/dsa_timecomplexity_insertionsort.php) [Quick Sort](https://www.w3schools.com/dsa/dsa_timecomplexity_quicksort.php) [Counting Sort](https://www.w3schools.com/dsa/dsa_timecomplexity_countsort.php) [Radix Sort](https://www.w3schools.com/dsa/dsa_timecomplexity_radixsort.php) [Merge Sort](https://www.w3schools.com/dsa/dsa_timecomplexity_mergesort.php) [Linear Search](https://www.w3schools.com/dsa/dsa_timecomplexity_linearsearch.php) [Binary Search](https://www.w3schools.com/dsa/dsa_timecomplexity_binarysearch.php) ## DSA Reference [DSA Euclidean Algorithm](https://www.w3schools.com/dsa/dsa_ref_euclidean_algorithm.php) [DSA Huffman Coding](https://www.w3schools.com/dsa/dsa_ref_huffman_coding.php) [DSA The Traveling Salesman](https://www.w3schools.com/dsa/dsa_ref_traveling_salesman.php) [DSA 0/1 Knapsack](https://www.w3schools.com/dsa/dsa_ref_knapsack.php) [DSA Memoization](https://www.w3schools.com/dsa/dsa_ref_memoization.php) [DSA Tabulation](https://www.w3schools.com/dsa/dsa_ref_tabulation.php) [DSA Dynamic Programming](https://www.w3schools.com/dsa/dsa_ref_dynamic_programming.php) [DSA Greedy Algorithms](https://www.w3schools.com/dsa/dsa_ref_greedy.php) ## DSA Examples [DSA Examples](https://www.w3schools.com/dsa/dsa_examples.php) [DSA Exercises](https://www.w3schools.com/dsa/dsa_exercises.php) [DSA Quiz](https://www.w3schools.com/dsa/dsa_quiz.php) [DSA Syllabus](https://www.w3schools.com/dsa/dsa_syllabus.php) [DSA Study Plan](https://www.w3schools.com/dsa/dsa_study_plan.php) [DSA Certificate](https://www.w3schools.com/dsa/dsa_exam.php) # DSA Queues [❮ Previous](https://www.w3schools.com/dsa/dsa_data_stacks.php) [Next ❯](https://www.w3schools.com/dsa/dsa_theory_hashtables.php) * ## Queues A queue is a data structure that can hold many elements. ![Out sign](https://www.w3schools.com/dsa/img_queuessign_out.svg) 3 2 1 4 ![In sign](https://www.w3schools.com/dsa/img_queuessign_in.svg) Result: enqueue() dequeue() peek() isEmpty() size() Think of a queue as people standing in line in a supermarket. The first person to stand in line is also the first who can pay and leave the supermarket. This way of organizing elements is called FIFO: First In First Out. Basic operations we can do on a queue are: - Enqueue: Adds a new element to the queue. - Dequeue: Removes and returns the first (front) element from the queue. - Peek: Returns the first element in the queue. - isEmpty: Checks if the queue is empty. - Size: Finds the number of elements in the queue. Experiment with these basic operations in the queue animation above. Queues can be implemented by using arrays or linked lists. Queues can be used to implement job scheduling for an office printer, order processing for e-tickets, or to create algorithms for breadth-first search in graphs. Queues are often mentioned together with Stacks, which is a similar data structure described on the [previous page](https://www.w3schools.com/dsa/dsa_data_stacks.php). * [REMOVE ADS](https://order.w3schools.com/plans) * ## Queue Implementation using Arrays To better understand the benefits with using arrays or linked lists to implement queues, you should check out [this page](https://www.w3schools.com/dsa/dsa_theory_linkedlists_memory.php) that explains how arrays and linked lists are stored in memory. This is how it looks like when we use an array as a queue: \[ 3, 2, 2, 2 \] Result: enqueue() dequeue() peek() isEmpty() size() Reasons to implement queues using arrays: - Memory Efficient: Array elements do not hold the next elements address like linked list nodes do. - Easier to implement and understand: Using arrays to implement queues require less code than using linked lists, and for this reason it is typically easier to understand as well. Reasons for not using arrays to implement queues: - Fixed size: An array occupies a fixed part of the memory. This means that it could take up more memory than needed, or if the array fills up, it cannot hold more elements. And resizing an array can be costly. - Shifting cost: Dequeue causes the first element in a queue to be removed, and the other elements must be shifted to take the removed elements' place. This is inefficient and can cause problems, especially if the queue is long. - Alternatives: Some programming languages have built-in data structures optimized for queue operations that are better than using arrays. Note: When using arrays in Python for this tutorial, we are really using the Python 'list' data type, but for the scope of this tutorial the 'list' data type can be used in the same way as an array. Learn more about Python lists [here](https://www.w3schools.com/python/python_lists.asp). Since Python lists has good support for functionality needed to implement queues, we start with creating a queue and do queue operations with just a few lines: ### Example Python: ``` queue = [] # Enqueue queue.append('A') queue.append('B') queue.append('C') print("Queue: ", queue) # Dequeue element = queue.pop(0) print("Dequeue: ", element) # Peek frontElement = queue[0] print("Peek: ", frontElement) # isEmpty isEmpty = not bool(queue) print("isEmpty: ", isEmpty) # Size print("Size: ", len(queue)) ``` [Try it Yourself »](https://www.w3schools.com/dsa/trydsa.php?filename=demo_queues_array1) But to explicitly create a data structure for queues, with basic operations, we should create a queue class instead. This way of creating queues in Python is also more similar to how queues can be created in other programming languages like C and Java. ### Example Python: ``` class Queue: def __init__(self): self.queue = [] def enqueue(self, element): self.queue.append(element) def dequeue(self): if self.isEmpty(): return "Queue is empty" return self.queue.pop(0) def peek(self): if self.isEmpty(): return "Queue is empty" return self.queue[0] def isEmpty(self): return len(self.queue) == 0 def size(self): return len(self.queue) # Create a queue myQueue = Queue() myQueue.enqueue('A') myQueue.enqueue('B') myQueue.enqueue('C') print("Queue: ", myQueue.queue) print("Dequeue: ", myQueue.dequeue()) print("Peek: ", myQueue.peek()) print("isEmpty: ", myQueue.isEmpty()) print("Size: ", myQueue.size()) ``` [Try it Yourself »](https://www.w3schools.com/dsa/trydsa.php?filename=demo_queues_array2) * ## Queue Implementation using Linked Lists Reasons for using linked lists to implement queues: - Dynamic size: The queue can grow and shrink dynamically, unlike with arrays. - No shifting: The front element of the queue can be removed (dequeue) without having to shift other elements in the memory. Reasons for not using linked lists to implement queues: - Extra memory: Each queue element must contain the address to the next element (the next linked list node). - Readability: The code might be harder to read and write for some because it is longer and more complex. This is how a queue can be implemented using a linked list. ### Example Python: ``` class Node: def __init__(self, data): self.data = data self.next = None class Queue: def __init__(self): self.front = None self.rear = None self.length = 0 def enqueue(self, element): new_node = Node(element) if self.rear is None: self.front = self.rear = new_node self.length += 1 return self.rear.next = new_node self.rear = new_node self.length += 1 def dequeue(self): if self.isEmpty(): return "Queue is empty" temp = self.front self.front = temp.next self.length -= 1 if self.front is None: self.rear = None return temp.data def peek(self): if self.isEmpty(): return "Queue is empty" return self.front.data def isEmpty(self): return self.length == 0 def size(self): return self.length def printQueue(self): temp = self.front while temp: print(temp.data, end=" ") temp = temp.next print() # Create a queue myQueue = Queue() myQueue.enqueue('A') myQueue.enqueue('B') myQueue.enqueue('C') print("Queue: ", end="") myQueue.printQueue() print("Dequeue: ", myQueue.dequeue()) print("Peek: ", myQueue.peek()) print("isEmpty: ", myQueue.isEmpty()) print("Size: ", myQueue.size()) ``` [Try it Yourself »](https://www.w3schools.com/dsa/trydsa.php?filename=demo_queues_linkedlist) * ## 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Readable Markdown	## DSA Queues * ## Queues A queue is a data structure that can hold many elements. ![Out sign](https://www.w3schools.com/dsa/img_queuessign_out.svg) 3 2 1 4 ![In sign](https://www.w3schools.com/dsa/img_queuessign_in.svg) Result: Think of a queue as people standing in line in a supermarket. The first person to stand in line is also the first who can pay and leave the supermarket. This way of organizing elements is called FIFO: First In First Out. Basic operations we can do on a queue are: - Enqueue: Adds a new element to the queue. - Dequeue: Removes and returns the first (front) element from the queue. - Peek: Returns the first element in the queue. - isEmpty: Checks if the queue is empty. - Size: Finds the number of elements in the queue. Experiment with these basic operations in the queue animation above. Queues can be implemented by using arrays or linked lists. Queues can be used to implement job scheduling for an office printer, order processing for e-tickets, or to create algorithms for breadth-first search in graphs. Queues are often mentioned together with Stacks, which is a similar data structure described on the [previous page](https://www.w3schools.com/dsa/dsa_data_stacks.php). * * ## Queue Implementation using Arrays To better understand the benefits with using arrays or linked lists to implement queues, you should check out [this page](https://www.w3schools.com/dsa/dsa_theory_linkedlists_memory.php) that explains how arrays and linked lists are stored in memory. This is how it looks like when we use an array as a queue: \[ 3, 2, 2, 2 \] Result: Reasons to implement queues using arrays: - Memory Efficient: Array elements do not hold the next elements address like linked list nodes do. - Easier to implement and understand: Using arrays to implement queues require less code than using linked lists, and for this reason it is typically easier to understand as well. Reasons for not using arrays to implement queues: - Fixed size: An array occupies a fixed part of the memory. This means that it could take up more memory than needed, or if the array fills up, it cannot hold more elements. And resizing an array can be costly. - Shifting cost: Dequeue causes the first element in a queue to be removed, and the other elements must be shifted to take the removed elements' place. This is inefficient and can cause problems, especially if the queue is long. - Alternatives: Some programming languages have built-in data structures optimized for queue operations that are better than using arrays. Note: When using arrays in Python for this tutorial, we are really using the Python 'list' data type, but for the scope of this tutorial the 'list' data type can be used in the same way as an array. Learn more about Python lists [here](https://www.w3schools.com/python/python_lists.asp). Since Python lists has good support for functionality needed to implement queues, we start with creating a queue and do queue operations with just a few lines: ### Example Python: ``` queue = [] # Enqueue queue.append('A') queue.append('B') queue.append('C') print("Queue: ", queue) # Dequeue element = queue.pop(0) print("Dequeue: ", element) # Peek frontElement = queue[0] print("Peek: ", frontElement) # isEmpty isEmpty = not bool(queue) print("isEmpty: ", isEmpty) # Size print("Size: ", len(queue)) ``` [Try it Yourself »](https://www.w3schools.com/dsa/trydsa.php?filename=demo_queues_array1) But to explicitly create a data structure for queues, with basic operations, we should create a queue class instead. This way of creating queues in Python is also more similar to how queues can be created in other programming languages like C and Java. ### Example Python: ``` class Queue: def __init__(self): self.queue = [] def enqueue(self, element): self.queue.append(element) def dequeue(self): if self.isEmpty(): return "Queue is empty" return self.queue.pop(0) def peek(self): if self.isEmpty(): return "Queue is empty" return self.queue[0] def isEmpty(self): return len(self.queue) == 0 def size(self): return len(self.queue) # Create a queue myQueue = Queue() myQueue.enqueue('A') myQueue.enqueue('B') myQueue.enqueue('C') print("Queue: ", myQueue.queue) print("Dequeue: ", myQueue.dequeue()) print("Peek: ", myQueue.peek()) print("isEmpty: ", myQueue.isEmpty()) print("Size: ", myQueue.size()) ``` [Try it Yourself »](https://www.w3schools.com/dsa/trydsa.php?filename=demo_queues_array2) * ## Queue Implementation using Linked Lists Reasons for using linked lists to implement queues: - Dynamic size: The queue can grow and shrink dynamically, unlike with arrays. - No shifting: The front element of the queue can be removed (dequeue) without having to shift other elements in the memory. Reasons for not using linked lists to implement queues: - Extra memory: Each queue element must contain the address to the next element (the next linked list node). - Readability: The code might be harder to read and write for some because it is longer and more complex. This is how a queue can be implemented using a linked list. ### Example Python: ``` class Node: def __init__(self, data): self.data = data self.next = None class Queue: def __init__(self): self.front = None self.rear = None self.length = 0 def enqueue(self, element): new_node = Node(element) if self.rear is None: self.front = self.rear = new_node self.length += 1 return self.rear.next = new_node self.rear = new_node self.length += 1 def dequeue(self): if self.isEmpty(): return "Queue is empty" temp = self.front self.front = temp.next self.length -= 1 if self.front is None: self.rear = None return temp.data def peek(self): if self.isEmpty(): return "Queue is empty" return self.front.data def isEmpty(self): return self.length == 0 def size(self): return self.length def printQueue(self): temp = self.front while temp: print(temp.data, end=" ") temp = temp.next print() # Create a queue myQueue = Queue() myQueue.enqueue('A') myQueue.enqueue('B') myQueue.enqueue('C') print("Queue: ", end="") myQueue.printQueue() print("Dequeue: ", myQueue.dequeue()) print("Peek: ", myQueue.peek()) print("isEmpty: ", myQueue.isEmpty()) print("Size: ", myQueue.size()) ``` [Try it Yourself »](https://www.w3schools.com/dsa/trydsa.php?filename=demo_queues_linkedlist) * * [![Get Certified Offer](https://www.w3schools.com/images/img_2026easter_fullaccess_300.webp)](https://campus.w3schools.com/products/w3schools-full-access-course)
Shard	40 (laksa)
Root Hash	10756103332767711440
Unparsed URL	com,w3schools!www,/dsa/dsa_data_queues.php s443