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class="vector-toc-link" href="#Examples_in_different_programming_languages"> <div class="vector-toc-text"> 3 Examples in different programming languages </div> </a> <ul id="toc-Examples_in_different_programming_languages-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Similar_constructs" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Similar_constructs"> <div class="vector-toc-text"> 4 Similar constructs </div> </a> <button aria-controls="toc-Similar_constructs-sublist" class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> Toggle Similar constructs subsection </button> <ul id="toc-Similar_constructs-sublist" class="vector-toc-list"> <li id="toc-Monad_comprehension" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Monad_comprehension"> <div class="vector-toc-text"> 4.1 Monad comprehension </div> </a> <ul id="toc-Monad_comprehension-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Set_comprehension" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Set_comprehension"> <div class="vector-toc-text"> 4.2 Set comprehension </div> </a> <ul id="toc-Set_comprehension-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Dictionary_comprehension" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Dictionary_comprehension"> <div class="vector-toc-text"> 4.3 Dictionary comprehension </div> </a> <ul id="toc-Dictionary_comprehension-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Parallel_list_comprehension" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Parallel_list_comprehension"> <div class="vector-toc-text"> 4.4 Parallel list comprehension </div> </a> <ul id="toc-Parallel_list_comprehension-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-XQuery_and_XPath" class="vector-toc-list-item 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lang="ca" hreflang="ca" data-title="Llista per comprensió" data-language-autonym="Català" data-language-local-name="Catalan" class="interlanguage-link-target">Català</a></li><li class="interlanguage-link interwiki-de mw-list-item"><a href="https://de.wikipedia.org/wiki/List_Comprehension" title="List Comprehension – German" lang="de" hreflang="de" data-title="List Comprehension" data-language-autonym="Deutsch" data-language-local-name="German" class="interlanguage-link-target">Deutsch</a></li><li class="interlanguage-link interwiki-et mw-list-item"><a href="https://et.wikipedia.org/wiki/Loendi_h%C3%B5lmamine" title="Loendi hõlmamine – Estonian" lang="et" hreflang="et" data-title="Loendi hõlmamine" data-language-autonym="Eesti" data-language-local-name="Estonian" class="interlanguage-link-target">Eesti</a></li><li class="interlanguage-link interwiki-fr mw-list-item"><a href="https://fr.wikipedia.org/wiki/Liste_en_compr%C3%A9hension" title="Liste en compréhension – French" lang="fr" hreflang="fr" data-title="Liste en compréhension" data-language-autonym="Français" data-language-local-name="French" class="interlanguage-link-target">Français</a></li><li class="interlanguage-link interwiki-ko mw-list-item"><a href="https://ko.wikipedia.org/wiki/%EB%A6%AC%EC%8A%A4%ED%8A%B8_%EC%BA%84%ED%94%84%EB%A6%AC%ED%97%A8%EC%85%98" title="리스트 캄프리헨션 – Korean" lang="ko" hreflang="ko" data-title="리스트 캄프리헨션" data-language-autonym="한국어" data-language-local-name="Korean" class="interlanguage-link-target">한국어</a></li><li class="interlanguage-link interwiki-it mw-list-item"><a href="https://it.wikipedia.org/wiki/Comprensione_di_lista" title="Comprensione di lista – Italian" lang="it" hreflang="it" data-title="Comprensione di lista" data-language-autonym="Italiano" data-language-local-name="Italian" class="interlanguage-link-target">Italiano</a></li><li class="interlanguage-link interwiki-nl mw-list-item"><a href="https://nl.wikipedia.org/wiki/Lijstcomprehensie" title="Lijstcomprehensie – Dutch" lang="nl" hreflang="nl" data-title="Lijstcomprehensie" data-language-autonym="Nederlands" data-language-local-name="Dutch" class="interlanguage-link-target">Nederlands</a></li><li class="interlanguage-link interwiki-ja mw-list-item"><a href="https://ja.wikipedia.org/wiki/%E3%83%AA%E3%82%B9%E3%83%88%E5%86%85%E5%8C%85%E8%A1%A8%E8%A8%98" title="リスト内包表記 – Japanese" lang="ja" hreflang="ja" data-title="リスト内包表記" data-language-autonym="日本語" data-language-local-name="Japanese" class="interlanguage-link-target">日本語</a></li><li class="interlanguage-link interwiki-pl mw-list-item"><a href="https://pl.wikipedia.org/wiki/Lista_sk%C5%82adana" title="Lista składana – Polish" lang="pl" hreflang="pl" data-title="Lista składana" data-language-autonym="Polski" data-language-local-name="Polish" class="interlanguage-link-target">Polski</a></li><li class="interlanguage-link interwiki-pt mw-list-item"><a href="https://pt.wikipedia.org/wiki/Compreens%C3%A3o_de_lista" title="Compreensão de lista – 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class="mw-redirect" title="Syntax of programming languages">syntactic</a> construct available in some <a href="/wiki/Programming_language" title="Programming language">programming languages</a> for creating a list based on existing <a href="/wiki/List_(computing)" class="mw-redirect" title="List (computing)">lists</a>. It follows the form of the mathematical <a href="/wiki/Set-builder_notation" title="Set-builder notation">set-builder notation</a> (set comprehension) as distinct from the use of <a href="/wiki/Map_(higher-order_function)" title="Map (higher-order function)">map</a> and <a href="/wiki/Filter_(higher-order_function)" title="Filter (higher-order function)">filter</a> functions. <meta property="mw:PageProp/toc" /> <div class="mw-heading mw-heading2"><h2 id="Overview">Overview</h2>[<a href="/w/index.php?title=List_comprehension&action=edit&section=1" title="Edit section: Overview">edit</a>]</div> Consider the following example in mathematical <a href="/wiki/Set-builder_notation" title="Set-builder notation">set-builder notation</a>. <dl><dd><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle S=\{2\cdot x\mid x\in \mathbb {N} ,\ x^{2}>3\}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mi>S</mi> <mo>=</mo> <mo fence="false" stretchy="false">{</mo> <mn>2</mn> <mo>⋅</mo> <mi>x</mi> <mo>∣</mo> <mi>x</mi> <mo>∈</mo> <mrow class="MJX-TeXAtom-ORD"> <mi mathvariant="double-struck">N</mi> </mrow> <mo>,</mo> <mtext> </mtext> <msup> <mi>x</mi> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> </mrow> </msup> <mo>></mo> <mn>3</mn> <mo fence="false" stretchy="false">}</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle S=\{2\cdot x\mid x\in \mathbb {N} ,\ x^{2}>3\}}</annotation> </semantics> </math><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/e6eafa5de185b3eeaab95c7ab27422b0b4d03e44" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:27.139ex; height:3.176ex;" alt="{\displaystyle S=\{2\cdot x\mid x\in \mathbb {N} ,\ x^{2}>3\}}"></dd></dl> or often <dl><dd><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle S=\{2\cdot x:x\in \mathbb {N} ,\ x^{2}>3\}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mi>S</mi> <mo>=</mo> <mo fence="false" stretchy="false">{</mo> <mn>2</mn> <mo>⋅</mo> <mi>x</mi> <mo>:</mo> <mi>x</mi> <mo>∈</mo> <mrow class="MJX-TeXAtom-ORD"> <mi mathvariant="double-struck">N</mi> </mrow> <mo>,</mo> <mtext> </mtext> <msup> <mi>x</mi> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> </mrow> </msup> <mo>></mo> <mn>3</mn> <mo fence="false" stretchy="false">}</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle S=\{2\cdot x:x\in \mathbb {N} ,\ x^{2}>3\}}</annotation> </semantics> </math><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/d2e592426d233755041915864dc34aa7588b2b45" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:27.139ex; height:3.176ex;" alt="{\displaystyle S=\{2\cdot x:x\in \mathbb {N} ,\ x^{2}>3\}}"></dd></dl> This can be read, "<math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle S}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mi>S</mi> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle S}</annotation> </semantics> </math><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/4611d85173cd3b508e67077d4a1252c9c05abca2" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:1.499ex; height:2.176ex;" alt="{\displaystyle S}"> is the set of all numbers "2 times <math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle x}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mi>x</mi> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle x}</annotation> </semantics> </math><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/87f9e315fd7e2ba406057a97300593c4802b53e4" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:1.33ex; height:1.676ex;" alt="{\displaystyle x}">" SUCH THAT <math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle x}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mi>x</mi> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle x}</annotation> </semantics> </math><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/87f9e315fd7e2ba406057a97300593c4802b53e4" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:1.33ex; height:1.676ex;" alt="{\displaystyle x}"> is an ELEMENT or MEMBER of the set of <a href="/wiki/Natural_number" title="Natural number">natural numbers</a> (<math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \mathbb {N} }"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mi mathvariant="double-struck">N</mi> </mrow> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle \mathbb {N} }</annotation> </semantics> </math><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/fdf9a96b565ea202d0f4322e9195613fb26a9bed" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:1.678ex; height:2.176ex;" alt="{\displaystyle \mathbb {N} }">), AND <math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle x}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mi>x</mi> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle x}</annotation> </semantics> </math><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/87f9e315fd7e2ba406057a97300593c4802b53e4" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:1.33ex; height:1.676ex;" alt="{\displaystyle x}"> squared is greater than <math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle 3}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mn>3</mn> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle 3}</annotation> </semantics> </math><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/991e33c6e207b12546f15bdfee8b5726eafbbb2f" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:1.162ex; height:2.176ex;" alt="{\displaystyle 3}">." The smallest natural number, x = 1, fails to satisfy the condition x2>3 (the condition 12>3 is false) so 2 ·1 is not included in S. The next natural number, 2, does satisfy the condition (22>3) as does every other natural number. Thus x consists of 2, 3, 4, 5... Since the set <var style="padding-right: 1px;">S</var> consists of all numbers "2 times x" it is given by S = {4, 6, 8, 10,...}. S is, in other words, the set of all even numbers greater than 2. In this annotated version of the example: <dl><dd><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle S=\{\underbrace {2\cdot x} _{\color {Violet}{\text{output expression}}}\mid \underbrace {x} _{\color {Violet}{\text{variable}}}\in \underbrace {\mathbb {N} } _{\color {Violet}{\text{input set}}},\ \underbrace {x^{2}>3} _{\color {Violet}{\text{predicate}}}\}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mi>S</mi> <mo>=</mo> <mo fence="false" stretchy="false">{</mo> <munder> <mrow class="MJX-TeXAtom-OP MJX-fixedlimits"> <munder> <mrow> <mn>2</mn> <mo>⋅</mo> <mi>x</mi> </mrow> <mo>⏟</mo> </munder> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mstyle mathcolor="#58429B"> <mrow class="MJX-TeXAtom-ORD"> <mtext>output expression</mtext> </mrow> </mstyle> </mrow> </munder> <mo>∣</mo> <munder> <mrow class="MJX-TeXAtom-OP MJX-fixedlimits"> <munder> <mi>x</mi> <mo>⏟</mo> </munder> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mstyle mathcolor="#58429B"> <mrow class="MJX-TeXAtom-ORD"> <mtext>variable</mtext> </mrow> </mstyle> </mrow> </munder> <mo>∈</mo> <munder> <mrow class="MJX-TeXAtom-OP MJX-fixedlimits"> <munder> <mrow class="MJX-TeXAtom-ORD"> <mi mathvariant="double-struck">N</mi> </mrow> <mo>⏟</mo> </munder> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mstyle mathcolor="#58429B"> <mrow class="MJX-TeXAtom-ORD"> <mtext>input set</mtext> </mrow> </mstyle> </mrow> </munder> <mo>,</mo> <mtext> </mtext> <munder> <mrow class="MJX-TeXAtom-OP MJX-fixedlimits"> <munder> <mrow> <msup> <mi>x</mi> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> </mrow> </msup> <mo>></mo> <mn>3</mn> </mrow> <mo>⏟</mo> </munder> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mstyle mathcolor="#58429B"> <mrow class="MJX-TeXAtom-ORD"> <mtext>predicate</mtext> </mrow> </mstyle> </mrow> </munder> <mo fence="false" stretchy="false">}</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle S=\{\underbrace {2\cdot x} _{\color {Violet}{\text{output expression}}}\mid \underbrace {x} _{\color {Violet}{\text{variable}}}\in \underbrace {\mathbb {N} } _{\color {Violet}{\text{input set}}},\ \underbrace {x^{2}>3} _{\color {Violet}{\text{predicate}}}\}}</annotation> </semantics> </math><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/611bbc7dd2005e4d52e287cdbf66cfb90782ccdb" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -4.171ex; width:44.912ex; height:6.509ex;" alt="{\displaystyle S=\{\underbrace {2\cdot x} _{\color {Violet}{\text{output expression}}}\mid \underbrace {x} _{\color {Violet}{\text{variable}}}\in \underbrace {\mathbb {N} } _{\color {Violet}{\text{input set}}},\ \underbrace {x^{2}>3} _{\color {Violet}{\text{predicate}}}\}}"></dd></dl> <ul><li><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle x}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mi>x</mi> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle x}</annotation> </semantics> </math><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/87f9e315fd7e2ba406057a97300593c4802b53e4" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:1.33ex; height:1.676ex;" alt="{\displaystyle x}"> is the variable representing members of an input set.</li> <li><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \mathbb {N} }"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mi mathvariant="double-struck">N</mi> </mrow> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle \mathbb {N} }</annotation> </semantics> </math><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/fdf9a96b565ea202d0f4322e9195613fb26a9bed" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:1.678ex; height:2.176ex;" alt="{\displaystyle \mathbb {N} }"> represents the input set, which in this example is the set of natural numbers</li> <li><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle x^{2}>3}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <msup> <mi>x</mi> <mrow class="MJX-TeXAtom-ORD"> <mn>2</mn> </mrow> </msup> <mo>></mo> <mn>3</mn> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle x^{2}>3}</annotation> </semantics> </math><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/66f5674402c8d90fd29f829d89d8ccc67efba4be" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:6.645ex; height:2.676ex;" alt="{\displaystyle x^{2}>3}"> is a <a href="/wiki/Predicate_(logic)" class="mw-redirect" title="Predicate (logic)">predicate</a> expression acting as a filter on members of the input set.</li> <li><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle 2\cdot x}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mn>2</mn> <mo>⋅</mo> <mi>x</mi> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle 2\cdot x}</annotation> </semantics> </math><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/31ff6145eb06b9724034b07d945719dd7fb50c0a" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:4.171ex; height:2.176ex;" alt="{\displaystyle 2\cdot x}"> is an output expression producing members of the new set from members of the input set that satisfy the predicate expression.</li> <li><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \{\}}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mo fence="false" stretchy="false">{</mo> <mo fence="false" stretchy="false">}</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle \{\}}</annotation> </semantics> </math><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/3e6f1caa524dfcc90158ad69a51b5f9577fe5f1f" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:2.325ex; height:2.843ex;" alt="{\displaystyle \{\}}"> braces indicate that the result is a set</li> <li><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \mid }"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mo>∣</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle \mid }</annotation> </semantics> </math><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/8f7b2136e276c4aec285a6c40b91180c16432b9b" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:0.647ex; height:2.843ex;" alt="{\displaystyle \mid }"> <math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle ,}"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mo>,</mo> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle ,}</annotation> </semantics> </math><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/b8fa4cba3a446de313920e16251756e27312b825" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.671ex; width:0.647ex; height:1.176ex;" alt="{\displaystyle ,}"> the vertical bar is read as "SUCH THAT". The bar and the colon ":" are used interchangeably.</li> <li>commas separate the predicates and can be read as "AND".</li></ul> A list comprehension has the same syntactic components to represent generation of a list in order from an input <a href="/wiki/List_(computing)" class="mw-redirect" title="List (computing)">list</a> or <a href="/wiki/Iterator" title="Iterator">iterator</a>: <ul><li>A variable representing members of an input list.</li> <li>An input list (or iterator).</li> <li>An optional predicate expression.</li> <li>And an output expression producing members of the output list from members of the input iterable that satisfy the predicate.</li></ul> The order of generation of members of the output list is based on the order of items in the input. In <a href="/wiki/Haskell_(programming_language)" class="mw-redirect" title="Haskell (programming language)">Haskell's</a> list comprehension syntax, this set-builder construct would be written similarly, as: <div class="mw-highlight mw-highlight-lang-haskell mw-content-ltr" dir="ltr"><pre>s = [ 2*x | x <- [0..], x^2 > 3 ] </pre></div> Here, the list <code>[0..]</code> represents <math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle \mathbb {N} }"> <semantics> <mrow class="MJX-TeXAtom-ORD"> <mstyle displaystyle="true" scriptlevel="0"> <mrow class="MJX-TeXAtom-ORD"> <mi mathvariant="double-struck">N</mi> </mrow> </mstyle> </mrow> <annotation encoding="application/x-tex">{\displaystyle \mathbb {N} }</annotation> </semantics> </math><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/fdf9a96b565ea202d0f4322e9195613fb26a9bed" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:1.678ex; height:2.176ex;" alt="{\displaystyle \mathbb {N} }">, <code>x^2>3</code> represents the predicate, and <code>2*x</code> represents the output expression. List comprehensions give results in a defined order (unlike the members of sets); and list comprehensions may <a href="/wiki/Generator_(computer_science)" class="mw-redirect" title="Generator (computer science)">generate</a> the members of a list in order, rather than produce the entirety of the list thus allowing, for example, the previous Haskell definition of the members of an infinite list. <div class="mw-heading mw-heading2"><h2 id="History">History</h2>[<a href="/w/index.php?title=List_comprehension&action=edit&section=2" title="Edit section: History">edit</a>]</div> The existence of related constructs predates the use of the term "List Comprehension". The <a href="/wiki/SETL" title="SETL">SETL</a> programming language (1969) has a set formation construct which is similar to list comprehensions. E.g., this code prints all prime numbers from 2 to <var style="padding-right: 1px;">N</var>: <pre>print([n in [2..N] | forall m in {2..n - 1} | n mod m > 0]); </pre> The <a href="/wiki/Computer_algebra_system" title="Computer algebra system">computer algebra system</a> <a href="/wiki/Axiom_(computer_algebra_system)" title="Axiom (computer algebra system)">AXIOM</a> (1973) has a similar construct that processes <a href="/wiki/Stream_(computing)" title="Stream (computing)">streams</a>. The first use of the term "comprehension" for such constructs was in <a href="/wiki/Rod_Burstall" title="Rod Burstall">Rod Burstall</a> and <a href="/wiki/John_Darlington" title="John Darlington">John Darlington</a>'s description of their functional programming language <a href="/wiki/NPL_programming_language" class="mw-redirect" title="NPL programming language">NPL</a> from 1977. In his retrospective "Some History of Functional Programming Languages",<a href="#cite_note-1">[1]</a> <a href="/wiki/David_Turner_(computer_scientist)" title="David Turner (computer scientist)">David Turner</a> recalls: <style data-mw-deduplicate="TemplateStyles:r1244412712">.mw-parser-output .templatequote{overflow:hidden;margin:1em 0;padding:0 32px}.mw-parser-output .templatequotecite{line-height:1.5em;text-align:left;margin-top:0}@media(min-width:500px){.mw-parser-output .templatequotecite{padding-left:1.6em}}</style><blockquote class="templatequote">NPL was implemented in POP2 by Burstall and used for Darlington’s work on program transformation (Burstall & Darlington 1977). The language was first order, strongly (but not polymorphically) typed, purely functional, call-by-value. It also had “set expressions” e.g. <dl><dd><pre>setofeven (X) <= <:x : x in X & even(x):>}}</pre></dd></dl></blockquote> In a footnote attached to the term "list comprehension", Turner also notes <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1244412712"><blockquote class="templatequote">I initially called these ZF expressions, a reference to <a href="/wiki/Zermelo%E2%80%93Fraenkel_set_theory" title="Zermelo–Fraenkel set theory">Zermelo–Fraenkel set theory</a> — it was <a href="/wiki/Philip_Wadler" title="Philip Wadler">Phil Wadler</a> who coined the better term list comprehension.</blockquote> Burstall and Darlington's work with NPL influenced many functional programming languages during the 1980s, but not all included list comprehensions. An exception was Turner's influential, pure, lazy, functional programming language <a href="/wiki/Miranda_programming_language" class="mw-redirect" title="Miranda programming language">Miranda</a>, released in 1985. The subsequently developed standard pure lazy functional language <a href="/wiki/Haskell_programming_language" class="mw-redirect" title="Haskell programming language">Haskell</a> includes many of Miranda's features, including list comprehensions. Comprehensions were proposed as a query notation for databases<a href="#cite_note-2">[2]</a> and were implemented in the <a href="/wiki/Heinrich_Kleisli" title="Heinrich Kleisli">Kleisli</a> database query language.<a href="#cite_note-3">[3]</a> <div class="mw-heading mw-heading2"><h2 id="Examples_in_different_programming_languages">Examples in different programming languages</h2>[<a href="/w/index.php?title=List_comprehension&action=edit&section=3" title="Edit section: Examples in different programming languages">edit</a>]</div> <style data-mw-deduplicate="TemplateStyles:r1236090951">.mw-parser-output .hatnote{font-style:italic}.mw-parser-output div.hatnote{padding-left:1.6em;margin-bottom:0.5em}.mw-parser-output .hatnote i{font-style:normal}.mw-parser-output .hatnote+link+.hatnote{margin-top:-0.5em}@media print{body.ns-0 .mw-parser-output .hatnote{display:none!important}}</style><div role="note" class="hatnote navigation-not-searchable">Main article: <a href="/wiki/Comparison_of_programming_languages_(list_comprehension)" title="Comparison of programming languages (list comprehension)">Comparison of programming languages (list comprehension)</a></div><div class="mw-highlight mw-highlight-lang-julia mw-content-ltr" dir="ltr"></div> <div class="mw-heading mw-heading2"><h2 id="Similar_constructs">Similar constructs</h2>[<a href="/w/index.php?title=List_comprehension&action=edit&section=4" title="Edit section: Similar constructs">edit</a>]</div> <div class="mw-heading mw-heading3"><h3 id="Monad_comprehension">Monad comprehension</h3>[<a href="/w/index.php?title=List_comprehension&action=edit&section=5" title="Edit section: Monad comprehension">edit</a>]</div> In Haskell, a <a href="/wiki/Monads_in_functional_programming#do-notation" class="mw-redirect" title="Monads in functional programming">monad comprehension</a> is a generalization of the list comprehension to other <a href="/wiki/Monads_in_functional_programming" class="mw-redirect" title="Monads in functional programming">monads in functional programming</a>. <div class="mw-heading mw-heading3"><h3 id="Set_comprehension">Set comprehension</h3>[<a href="/w/index.php?title=List_comprehension&action=edit&section=6" title="Edit section: Set comprehension">edit</a>]</div> The <a href="/wiki/Python_(programming_language)" title="Python (programming language)">Python</a> language introduces syntax for <a href="/wiki/Set_(computer_science)" class="mw-redirect" title="Set (computer science)">set</a> comprehensions starting in version 2.7. Similar in form to list comprehensions, set comprehensions generate Python sets instead of lists. <div class="mw-highlight mw-highlight-lang-pycon mw-content-ltr" dir="ltr"><pre>>>> s = {v for v in 'ABCDABCD' if v not in 'CB'} >>> print(s) {'A', 'D'} >>> type(s) <class 'set'> >>> </pre></div> <a href="/wiki/Racket_(programming_language)" title="Racket (programming language)">Racket</a> set comprehensions generate Racket sets instead of lists. <div class="mw-highlight mw-highlight-lang-scheme mw-content-ltr" dir="ltr"><pre>(for/set ([v "ABCDABCD"] #:unless (member v (string->list "CB"))) v)) </pre></div> <div class="mw-heading mw-heading3"><h3 id="Dictionary_comprehension">Dictionary comprehension</h3>[<a href="/w/index.php?title=List_comprehension&action=edit&section=7" title="Edit section: Dictionary comprehension">edit</a>]</div> The <a href="/wiki/Python_(programming_language)" title="Python (programming language)">Python</a> language introduced a new syntax for <a href="/wiki/Associative_array" title="Associative array">dictionary</a> comprehensions in version 2.7, similar in form to list comprehensions but which generate Python <a rel="nofollow" class="external text" href="https://docs.python.org/library/stdtypes.html#dict">dicts</a> instead of lists. <div class="mw-highlight mw-highlight-lang-pycon mw-content-ltr" dir="ltr"><pre>>>> s = {key: val for key, val in enumerate('ABCD') if val not in 'CB'} >>> s {0: 'A', 3: 'D'} >>> </pre></div> Racket hash table comprehensions generate Racket hash tables (one implementation of the Racket dictionary type). <div class="mw-highlight mw-highlight-lang-scheme mw-content-ltr" dir="ltr"><pre>(for/hash ([(val key) (in-indexed "ABCD")] #:unless (member val (string->list "CB"))) (values key val)) </pre></div> <div class="mw-heading mw-heading3"><h3 id="Parallel_list_comprehension">Parallel list comprehension</h3>[<a href="/w/index.php?title=List_comprehension&action=edit&section=8" title="Edit section: Parallel list comprehension">edit</a>]</div> The <a href="/wiki/Glasgow_Haskell_Compiler" title="Glasgow Haskell Compiler">Glasgow Haskell Compiler</a> has an extension called parallel list comprehension (also known as zip-comprehension) that permits multiple independent branches of qualifiers within the list comprehension syntax. Whereas qualifiers separated by commas are dependent ("nested"), qualifier branches separated by pipes are evaluated in parallel (this does not refer to any form of multithreadedness: it merely means that the branches are <a href="/wiki/Map_(higher-order_function)" title="Map (higher-order function)">zipped</a>). <div class="mw-highlight mw-highlight-lang-haskell mw-content-ltr" dir="ltr"><pre>-- regular list comprehension a = [(x,y) | x <- [1..5], y <- [3..5]] -- [(1,3),(1,4),(1,5),(2,3),(2,4) ... -- zipped list comprehension b = [(x,y) | (x,y) <- zip [1..5] [3..5]] -- [(1,3),(2,4),(3,5)] -- parallel list comprehension c = [(x,y) | x <- [1..5] | y <- [3..5]] -- [(1,3),(2,4),(3,5)] </pre></div> Racket's comprehensions standard library contains parallel and nested versions of its comprehensions, distinguished by "for" vs "for*" in the name. For example, the vector comprehensions "for/vector" and "for*/vector" create vectors by parallel versus nested iteration over sequences. The following is Racket code for the Haskell list comprehension examples. <div class="mw-highlight mw-highlight-lang-scheme mw-content-ltr" dir="ltr"><pre>> (for*/list ([x (in-range 1 6)] [y (in-range 3 6)]) (list x y)) '((1 3) (1 4) (1 5) (2 3) (2 4) (2 5) (3 3) (3 4) (3 5) (4 3) (4 4) (4 5) (5 3) (5 4) (5 5)) > (for/list ([x (in-range 1 6)] [y (in-range 3 6)]) (list x y)) '((1 3) (2 4) (3 5)) </pre></div> In Python, we could do as follows: <div class="mw-highlight mw-highlight-lang-pycon mw-content-ltr" dir="ltr"><pre>>>> # regular list comprehension >>> a = [(x, y) for x in range(1, 6) for y in range(3, 6)] [(1, 3), (1, 4), (1, 5), (2, 3), (2, 4), ... >>> >>> # parallel/zipped list comprehension >>> b = [x for x in zip(range(1, 6), range(3, 6))] [(1, 3), (2, 4), (3, 5)] </pre></div> In Julia, practically the same results can be achieved as follows: <div class="mw-highlight mw-highlight-lang-julia mw-content-ltr" dir="ltr"><pre># regular array comprehension >>> a = [(x, y) for x in 1:5 for y in 3:5] # parallel/zipped array comprehension >>> b = [x for x in zip(1:3, 3:5)] </pre></div> with the only difference that instead of lists, in Julia, we have arrays. <div class="mw-heading mw-heading3"><h3 id="XQuery_and_XPath">XQuery and XPath</h3>[<a href="/w/index.php?title=List_comprehension&action=edit&section=9" title="Edit section: XQuery and XPath">edit</a>]</div> Like the original NPL use, these are fundamentally database access languages. This makes the comprehension concept more important, because it is computationally infeasible to retrieve the entire list and operate on it (the initial 'entire list' may be an entire XML database). In XPath, the expression: <div class="mw-highlight mw-highlight-lang-xquery mw-content-ltr" dir="ltr"><pre>/library/book//paragraph[@style='first-in-chapter'] </pre></div> is conceptually evaluated as a series of "steps" where each step produces a list and the next step applies a filter function to each element in the previous step's output.<a href="#cite_note-4">[4]</a> In XQuery, full XPath is available, but <a href="/wiki/FLWOR" title="FLWOR">FLWOR</a> statements are also used, which is a more powerful comprehension construct.<a href="#cite_note-5">[5]</a> <div class="mw-highlight mw-highlight-lang-xquery mw-content-ltr" dir="ltr"><pre>for $b in //book where $b[@pages < 400] order by $b//title return <shortBook> <title>{$b//title}</title> <firstPara>{($book//paragraph)[1]}</firstPara> </shortBook> </pre></div> Here the XPath //book is evaluated to create a sequence (aka list); the where clause is a functional "filter", the order by sorts the result, and the <code class="nowrap" style=""><shortBook>...</shortBook></code> XML snippet is actually an <a href="/wiki/Anonymous_function" title="Anonymous function">anonymous function</a> that builds/transforms XML for each element in the sequence using the 'map' approach found in other functional languages. So, in another functional language the above FLWOR statement may be implemented like this: <div class="mw-highlight mw-highlight-lang-xquery mw-content-ltr" dir="ltr"><pre>map( newXML(shortBook, newXML(title, $1.title), newXML(firstPara, $1...)) filter( lt($1.pages, 400), xpath(//book) ) ) </pre></div> <div class="mw-heading mw-heading3"><h3 id="LINQ_in_C#">LINQ in C#</h3>[<a href="/w/index.php?title=List_comprehension&action=edit&section=10" title="Edit section: LINQ in C#">edit</a>]</div> <a href="/wiki/C_Sharp_(programming_language)" title="C Sharp (programming language)">C#</a> 3.0 has a group of related features called <a href="/wiki/Language_Integrated_Query" title="Language Integrated Query">LINQ</a>, which defines a set of query operators for manipulating object enumerations. <div class="mw-highlight mw-highlight-lang-csharp mw-content-ltr" dir="ltr"><pre>var s = Enumerable.Range(0, 100).Where(x => x * x > 3).Select(x => x * 2); </pre></div> It also offers an alternative comprehension syntax, reminiscent of SQL: <div class="mw-highlight mw-highlight-lang-csharp mw-content-ltr" dir="ltr"><pre>var s = from x in Enumerable.Range(0, 100) where x * x > 3 select x * 2; </pre></div> LINQ provides a capability over typical list comprehension implementations. When the root object of the comprehension implements the <code>IQueryable</code> interface, rather than just executing the chained methods of the comprehension, the entire sequence of commands are converted into an <a href="/wiki/Abstract_syntax_tree" title="Abstract syntax tree">abstract syntax tree</a> (AST) object, which is passed to the IQueryable object to interpret and execute. This allows, amongst other things, for the IQueryable to <ul><li>rewrite an incompatible or inefficient comprehension</li> <li>translate the AST into another query language (e.g. SQL) for execution</li></ul> <div class="mw-heading mw-heading3"><h3 id="C++">C++</h3>[<a href="/w/index.php?title=List_comprehension&action=edit&section=11" title="Edit section: C++">edit</a>]</div> C++ does not have any language features directly supporting list comprehensions but <a href="/wiki/Operator_overloading" title="Operator overloading">operator overloading</a> (e.g., overloading <code>|</code>, <code>>></code>, <code>>>=</code>) has been used successfully to provide expressive syntax for "embedded" query <a href="/wiki/Domain-specific_language" title="Domain-specific language">domain-specific languages</a> (DSL). Alternatively, list comprehensions can be constructed using the <a href="/wiki/Erase-remove_idiom" class="mw-redirect" title="Erase-remove idiom">erase-remove idiom</a> to select elements in a container and the STL algorithm for_each to transform them. <div class="mw-highlight mw-highlight-lang-cpp mw-content-ltr" dir="ltr"><pre>#include <algorithm> #include <list> #include <numeric> using namespace std; template<class C, class P, class T> C comprehend(C&& source, const P& predicate, const T& transformation) { // initialize destination C d = forward<C>(source); // filter elements d.erase(remove_if(begin(d), end(d), predicate), end(d)); // apply transformation for_each(begin(d), end(d), transformation); return d; } int main() { list<int> range(10); // range is a list of 10 elements, all zero iota(begin(range), end(range), 1); // range now contains 1, 2, ..., 10 list<int> result = comprehend( range, [](int x) { return x * x <= 3; }, [](int &x) { x *= 2; }); // result now contains 4, 6, ..., 20 } </pre></div> There is some effort in providing C++ with list-comprehension constructs/syntax similar to the set builder notation. <ul><li>In <a href="/wiki/Boost_C%2B%2B_Libraries" class="mw-redirect" title="Boost C++ Libraries">Boost</a>. Range <a rel="nofollow" class="external autonumber" href="http://www.boost.org/libs/range">[1]</a> library there is a notion of adaptors <a rel="nofollow" class="external autonumber" href="http://www.boost.org/libs/range/doc/html/range/reference/adaptors.html">[2]</a> that can be applied to any range and do filtering, transformation etc. With this library, the original Haskell example would look like (using Boost.Lambda <a rel="nofollow" class="external autonumber" href="http://www.boost.org/libs/lambda">[3]</a> for anonymous filtering and transforming functions) (<a rel="nofollow" class="external text" href="http://codepad.org/y4bpgLJu">Full example</a>):<div class="mw-highlight mw-highlight-lang-cpp mw-content-ltr" dir="ltr"><pre>counting_range(1,10) | filtered( _1*_1 > 3 ) | transformed(ret<int>( _1*2 )) </pre></div></li> <li>This<a href="#cite_note-6">[6]</a> implementation uses a macro and overloads the << operator. It evaluates any expression valid inside an 'if', and any variable name may be chosen. It's not <a href="/wiki/Thread_safety" title="Thread safety">threadsafe</a>, however. Usage example:</li></ul> <div class="mw-highlight mw-highlight-lang-cpp mw-content-ltr" dir="ltr"><pre>list<int> N; list<double> S; for (int i = 0; i < 10; i++) N.push_back(i); S << list_comprehension(3.1415 * x, x, N, x * x > 3) </pre></div> <ul><li>This<a href="#cite_note-7">[7]</a> implementation provides head/tail slicing using classes and operator overloading, and the | operator for filtering lists (using functions). Usage example:</li></ul> <div class="mw-highlight mw-highlight-lang-cpp mw-content-ltr" dir="ltr"><pre>bool even(int x) { return x % 2 == 0; } bool x2(int &x) { x *= 2; return true; } list<int> l, t; int x, y; for (int i = 0; i < 10; i++) l.push_back(i); (x, t) = l | x2; (t, y) = t; t = l < 9; t = t < 7 | even | x2; </pre></div> <ul><li>Language for Embedded Query and Traversal (LEESA<a href="#cite_note-8">[8]</a>) is an embedded DSL in C++ that implements X-Path-like queries using operator overloading. The queries are executed on richly typed xml trees obtained using xml-to-c++ binding from an XSD. There is absolutely no string encoding. Even the names of the xml tags are classes and therefore, there is no way for typos. If a LEESA expression forms an incorrect path that does not exist in the data model, the C++ compiler will reject the code. Consider a catalog xml.</li></ul> <div class="mw-highlight mw-highlight-lang-xml mw-content-ltr" dir="ltr"><pre><catalog> <book> <title>Hamlet</title> <price>9.99</price> <author> <name>William Shakespeare</name> <country>England</country> </author> </book> <book>...</book> ... </catalog> </pre></div> LEESA provides <code>>></code> for XPath's / separator. XPath's // separator that "skips" intermediate nodes in the tree is implemented in LEESA using what's known as Strategic Programming. In the example below, catalog_, book_, author_, and name_ are instances of catalog, book, author, and name classes, respectively. <div class="mw-highlight mw-highlight-lang-cpp mw-content-ltr" dir="ltr"><pre>// Equivalent X-Path: "catalog/book/author/name" std::vector<name> author_names = evaluate(root, catalog_ >> book_ >> author_ >> name_); // Equivalent X-Path: "catalog//name" std::vector<name> author_names = evaluate(root, catalog_ >> DescendantsOf(catalog_, name_)); // Equivalent X-Path: "catalog//author[country=="England"]" std::vector<name> author_names = evaluate(root, catalog_ >> DescendantsOf(catalog_, author_) >> Select(author_, [](const author & a) { return a.country() == "England"; }) >> name_); </pre></div> <div class="mw-heading mw-heading2"><h2 id="See_also">See also</h2>[<a href="/w/index.php?title=List_comprehension&action=edit&section=12" title="Edit section: See also">edit</a>]</div> <ul><li><a href="/wiki/Set-builder_notation" title="Set-builder notation">Set-builder notation</a></li> <li>The <a href="/wiki/Select_(SQL)" title="Select (SQL)">SELECT</a> statement together with its FROM and WHERE clauses in <a href="/wiki/SQL#Queries" title="SQL">SQL</a></li></ul> <div class="mw-heading mw-heading2"><h2 id="Notes_and_references">Notes and references</h2>[<a href="/w/index.php?title=List_comprehension&action=edit&section=13" title="Edit section: Notes and references">edit</a>]</div> <style data-mw-deduplicate="TemplateStyles:r1239543626">.mw-parser-output .reflist{margin-bottom:0.5em;list-style-type:decimal}@media screen{.mw-parser-output .reflist{font-size:90%}}.mw-parser-output .reflist .references{font-size:100%;margin-bottom:0;list-style-type:inherit}.mw-parser-output .reflist-columns-2{column-width:30em}.mw-parser-output .reflist-columns-3{column-width:25em}.mw-parser-output .reflist-columns{margin-top:0.3em}.mw-parser-output .reflist-columns ol{margin-top:0}.mw-parser-output .reflist-columns li{page-break-inside:avoid;break-inside:avoid-column}.mw-parser-output .reflist-upper-alpha{list-style-type:upper-alpha}.mw-parser-output .reflist-upper-roman{list-style-type:upper-roman}.mw-parser-output .reflist-lower-alpha{list-style-type:lower-alpha}.mw-parser-output .reflist-lower-greek{list-style-type:lower-greek}.mw-parser-output .reflist-lower-roman{list-style-type:lower-roman}</style><div class="reflist"> <div class="mw-references-wrap"><ol class="references"> <li id="cite_note-1"><a href="#cite_ref-1">^</a> <style data-mw-deduplicate="TemplateStyles:r1238218222">.mw-parser-output cite.citation{font-style:inherit;word-wrap:break-word}.mw-parser-output .citation q{quotes:"\"""\"""'""'"}.mw-parser-output .citation:target{background-color:rgba(0,127,255,0.133)}.mw-parser-output .id-lock-free.id-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/6/65/Lock-green.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-limited.id-lock-limited a,.mw-parser-output .id-lock-registration.id-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/d/d6/Lock-gray-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-subscription.id-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/a/aa/Lock-red-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .cs1-ws-icon a{background:url("//upload.wikimedia.org/wikipedia/commons/4/4c/Wikisource-logo.svg")right 0.1em center/12px no-repeat}body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-free a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-limited a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-registration a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-subscription a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .cs1-ws-icon a{background-size:contain;padding:0 1em 0 0}.mw-parser-output .cs1-code{color:inherit;background:inherit;border:none;padding:inherit}.mw-parser-output .cs1-hidden-error{display:none;color:var(--color-error,#d33)}.mw-parser-output .cs1-visible-error{color:var(--color-error,#d33)}.mw-parser-output .cs1-maint{display:none;color:#085;margin-left:0.3em}.mw-parser-output .cs1-kern-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right{padding-right:0.2em}.mw-parser-output .citation .mw-selflink{font-weight:inherit}@media screen{.mw-parser-output .cs1-format{font-size:95%}html.skin-theme-clientpref-night .mw-parser-output .cs1-maint{color:#18911f}}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .cs1-maint{color:#18911f}}</style><cite id="CITEREFTurner2012" class="citation conference cs1">Turner, David (2012). <a rel="nofollow" class="external text" href="https://www.cs.kent.ac.uk/people/staff/dat/tfp12/tfp12.pdf">"Some history of functional programming languages"</a> (PDF). 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Archived from <a rel="nofollow" class="external text" href="https://www.w3schools.com/XQuery/xquery_flwor.asp">the original</a> on 2011-10-08.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=unknown&rft.jtitle=W3Schools&rft.atitle=XQuery+FLWOR+Expressions&rft_id=https%3A%2F%2Fwww.w3schools.com%2FXQuery%2Fxquery_flwor.asp&rfr_id=info%3Asid%2Fen.wikipedia.org%3AList+comprehension" class="Z3988"> </li> <li id="cite_note-6"><a href="#cite_ref-6">^</a> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://web.archive.org/web/20110821211656/http://mfoliveira.org/blog/2011/01/04/simple-list-comprehension-in-cp-using-preprocessor-macros/">"Single-variable List Comprehension in C++ using Preprocessor Macros"</a>. 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Archived from <a rel="nofollow" class="external text" href="http://www.tedunangst.com/listcc.html">the original</a> on 2017-07-07. Retrieved 2011-01-09.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=C%2B%2B+list+comprehensions&rft_id=http%3A%2F%2Fwww.tedunangst.com%2Flistcc.html&rfr_id=info%3Asid%2Fen.wikipedia.org%3AList+comprehension" class="Z3988"> </li> <li id="cite_note-8"><a href="#cite_ref-8">^</a> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://www.dre.vanderbilt.edu/LEESA/">"Language for Embedded Query and Traversal (LEESA)"</a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=Language+for+Embedded+Query+and+Traversal+%28LEESA%29&rft_id=http%3A%2F%2Fwww.dre.vanderbilt.edu%2FLEESA%2F&rfr_id=info%3Asid%2Fen.wikipedia.org%3AList+comprehension" class="Z3988"> </li> </ol></div></div> <ul><li><a rel="nofollow" class="external text" href="https://web.archive.org/web/20050125080818/http://ftp.sunet.se/foldoc/foldoc.cgi?list+comprehension">List Comprehension</a> in The Free On-line Dictionary of Computing, Editor Denis Howe.</li> <li><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFWadler1990" class="citation conference cs1">Wadler, Philip (1990). <a rel="nofollow" class="external text" href="http://citeseer.ist.psu.edu/wadler92comprehending.html">"Comprehending Monads"</a>. Proceedings of the 1990 ACM Conference on LISP and Functional Programming, Nice.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=conference&rft.atitle=Comprehending+Monads&rft.btitle=Proceedings+of+the+1990+ACM+Conference+on+LISP+and+Functional+Programming%2C+Nice&rft.date=1990&rft.aulast=Wadler&rft.aufirst=Philip&rft_id=http%3A%2F%2Fciteseer.ist.psu.edu%2Fwadler92comprehending.html&rfr_id=info%3Asid%2Fen.wikipedia.org%3AList+comprehension" class="Z3988"></li></ul> <div class="mw-heading mw-heading2"><h2 id="External_links">External links</h2>[<a href="/w/index.php?title=List_comprehension&action=edit&section=14" title="Edit section: External links">edit</a>]</div> <ul><li>SQL-like set operations with list comprehension one-liners in the <a rel="nofollow" class="external text" href="http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/159974">Python Cookbook</a></li> <li><a rel="nofollow" class="external text" href="http://lambda-the-ultimate.org/classic/message11326.html">Discussion on list comprehensions in Scheme and related constructs</a></li> <li><a rel="nofollow" class="external text" href="http://langexplr.blogspot.com/2007/02/list-comprehensions-across-languages_18.html">List Comprehensions across languages</a></li></ul> <div class="mw-heading mw-heading3"><h3 id="Axiom">Axiom</h3>[<a href="/w/index.php?title=List_comprehension&action=edit&section=15" title="Edit section: Axiom">edit</a>]</div> <ul><li><a rel="nofollow" class="external text" href="https://web.archive.org/web/20051018040438/http://page.axiom-developer.org/zope/mathaction/Streams">Axiom stream examples</a></li></ul> <div class="mw-heading mw-heading3"><h3 id="Clojure">Clojure</h3>[<a href="/w/index.php?title=List_comprehension&action=edit&section=16" title="Edit section: Clojure">edit</a>]</div> <ul><li><a rel="nofollow" class="external text" href="https://clojuredocs.org/clojure.core/for">Clojure API documentation - for macro</a></li></ul> <div class="mw-heading mw-heading3"><h3 id="Common_Lisp">Common Lisp</h3>[<a href="/w/index.php?title=List_comprehension&action=edit&section=17" title="Edit section: Common Lisp">edit</a>]</div> <ul><li><a rel="nofollow" class="external text" href="http://rali.iro.umontreal.ca/rali/?q=en/node/886">Implementation of a Lisp comprehension macro</a> by Guy Lapalme</li></ul> <div class="mw-heading mw-heading3"><h3 id="Haskell">Haskell</h3>[<a href="/w/index.php?title=List_comprehension&action=edit&section=18" title="Edit section: Haskell">edit</a>]</div> <ul><li>The Haskell 98 Report, chapter <a rel="nofollow" class="external text" href="http://haskell.org/onlinereport/exps.html#list-comprehensions">3.11 List Comprehensions</a>.</li> <li>The Glorious Glasgow Haskell Compilation System User's Guide, chapter <a rel="nofollow" class="external text" href="https://web.archive.org/web/20051129140339/http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#parallel-list-comprehensions">7.3.4 Parallel List Comprehensions</a>.</li> <li>The Hugs 98 User's Guide, chapter <a rel="nofollow" class="external text" href="https://web.archive.org/web/20140515114545/http://cvs.haskell.org/Hugs/pages/users_guide/hugs-ghc.html#ZIP-COMPREHENSION">5.1.2 Parallel list comprehensions (a.k.a. zip-comprehensions)</a>.</li></ul> <div class="mw-heading mw-heading3"><h3 id="OCaml">OCaml</h3>[<a href="/w/index.php?title=List_comprehension&action=edit&section=19" title="Edit section: OCaml">edit</a>]</div> <ul><li><a rel="nofollow" class="external text" href="http://batteries.forge.ocamlcore.org/">OCaml Batteries Included</a></li> <li><a rel="nofollow" class="external text" href="http://batteries.forge.ocamlcore.org/doc.preview:batteries-alpha3/html/extensions.html">Language extensions introduced in OCaml Batteries Included</a></li></ul> <div class="mw-heading mw-heading3"><h3 id="Python">Python</h3>[<a href="/w/index.php?title=List_comprehension&action=edit&section=20" title="Edit section: Python">edit</a>]</div> <ul><li>The Python Tutorial, <a rel="nofollow" class="external text" href="https://docs.python.org/tutorial/datastructures.html#list-comprehensions">List Comprehensions</a>.</li> <li>Python Language Reference, <a rel="nofollow" class="external text" href="https://docs.python.org/reference/expressions.html#list-displays">List displays</a>.</li> <li>Python Enhancement Proposal <a rel="nofollow" class="external text" href="https://www.python.org/peps/pep-0202.html">PEP 202: List Comprehensions</a>.</li> <li>Python Language Reference, <a rel="nofollow" class="external text" href="https://docs.python.org/reference/expressions.html#generator-expressions">Generator expressions</a>.</li> <li>Python Enhancement Proposal <a rel="nofollow" class="external text" href="https://python.org/peps/pep-0289.html">PEP 289: Generator Expressions</a>.</li></ul>    </div><noscript><img src="https://login.wikimedia.org/wiki/Special:CentralAutoLogin/start?useformat=desktop&type=1x1&usesul3=0" alt="" width="1" height="1" style="border: none; position: absolute;"></noscript> <div class="printfooter" data-nosnippet="">Retrieved from "<a dir="ltr" href="https://en.wikipedia.org/w/index.php?title=List_comprehension&oldid=1228787071">https://en.wikipedia.org/w/index.php?title=List_comprehension&oldid=1228787071</a>"</div></div> <div id="catlinks" class="catlinks" data-mw="interface"><div id="mw-normal-catlinks" class="mw-normal-catlinks"><a href="/wiki/Help:Category" title="Help:Category">Category</a>: <ul><li><a href="/wiki/Category:Programming_constructs" title="Category:Programming constructs">Programming constructs</a></li></ul></div><div id="mw-hidden-catlinks" class="mw-hidden-catlinks mw-hidden-cats-hidden">Hidden categories: <ul><li><a href="/wiki/Category:Articles_with_short_description" 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