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<!DOCTYPE html> <html lang="en"> <head> <title>#? -- check existence of value in a list, hash table, database, or string</title> <meta content="text/html; charset=utf-8" http-equiv="Content-Type"> <link type="text/css" rel="stylesheet" href="../../../../Macaulay2/Style/doc.css"> <link rel="stylesheet" href="../../../../Macaulay2/Style/katex/katex.min.css"> <script defer="defer" src="../../../../Macaulay2/Style/katex/katex.min.js"></script> <script defer="defer" src="../../../../Macaulay2/Style/katex/contrib/auto-render.min.js"></script> <script> var macros = { "\\break": "\\\\", "\\ZZ": "\\mathbb{Z}", "\\NN": "\\mathbb{N}", "\\QQ": "\\mathbb{Q}", "\\RR": "\\mathbb{R}", "\\CC": "\\mathbb{C}", "\\PP": "\\mathbb{P}" }, delimiters = [ { left: "$$", right: "$$", display: true}, { left: "\\[", right: "\\]", display: true}, { left: "$", right: "$", display: false}, { left: "\\(", right: "\\)", display: false} ], ignoredTags = [ "kbd", "var", "samp", "script", "noscript", "style", "textarea", "pre", "code", "option" ]; document.addEventListener("DOMContentLoaded", function() { renderMathInElement(document.body, { delimiters: delimiters, macros: macros, ignoredTags: ignoredTags, trust: true }); }); </script> <style>.katex { font-size: 1em; }</style> <script defer="defer" src="../../../../Macaulay2/Style/katex/contrib/copy-tex.min.js"></script> <script defer="defer" src="../../../../Macaulay2/Style/katex/contrib/render-a11y-string.min.js"></script> <script src="../../../../Macaulay2/Style/prism.js"></script> <script>var current_version = '1.25.06';</script> <script src="../../../../Macaulay2/Style/version-select.js"></script> <link type="image/x-icon" rel="icon" href="../../../../Macaulay2/Style/icon.gif"> </head> <body> <div id="buttons"> <div> <a href="https://macaulay2.com/">Macaulay2</a> <span id="version-select-container"></span> » <a title="Macaulay2 documentation" href="index.html">Documentation </a> <br><a href="_packages_spprovided_spwith_sp__Macaulay2.html">Packages</a> » <span><a title="Macaulay2 documentation" href="index.html">Macaulay2Doc</a> » <a href="___The_sp__Macaulay2_splanguage.html">The Macaulay2 language</a> » <a href="_operators.html">operators</a> » <a title="check existence of value in a list, hash table, database, or string" href="__sh_qu.html">#?</a></span> </div> <div class="right"> <form method="get" action="https://www.google.com/search"> <input placeholder="Search" type="text" name="q" value=""> <input type="hidden" name="q" value="site:macaulay2.com/doc"> </form> <a href="_%21.html">next</a> | <a href="__sh.html">previous</a> | <a href="_%21.html">forward</a> | <a href="__sh.html">backward</a> | <a href="_operators.html">up</a> | <a href="master.html">index</a> | <a href="toc.html">toc</a> </div> </div> <hr> <div> <h1>#? -- check existence of value in a list, hash table, database, or string</h1> <ul> <li> <dl class="element"> <dt>Usage: </dt> <dd><code class="language-macaulay2">x#?i</code></dd> </dl> </li> <li>Inputs: <ul> <li><span><span class="tt">x</span>, <span>a <a title="the class of all lists -- {...}" href="___List.html">list</a></span>, <span>a <a title="the class of all hash tables" href="___Hash__Table.html">hash table</a></span>, <span>a <a title="the class of all database files" href="___Database.html">database</a></span>, or <span>a <a title="the class of all strings" href="___String.html">string</a></span>, </span></li> <li><span><span class="tt">i</span>, an index or key</span></li> </ul> </li> <li>Outputs: <ul> <li><span><span>a <a title="the class of boolean values" href="___Boolean.html">Boolean value</a></span>, whether or not <span class="tt">x</span> contains an element with index or key <span class="tt">i</span></span></li> </ul> </li> </ul> <div> <h2>Description</h2> <div> <p>If <span class="tt">x</span> is a list, <span class="tt">x#?i</span> tells whether there is an <span class="tt">i</span>th element of <span class="tt">x</span>. The entries of the list are numbered starting with 0. If <span class="tt">i</span> is negative, then the entries are numbered ending with -1.</p> </div> <table class="examples"> <tr> <td> <pre><code class="language-macaulay2">i1 : L = {a, b, c, b, a};</code></pre> </td> </tr> <tr> <td> <pre><code class="language-macaulay2">i2 : L#?2 o2 = true</code></pre> </td> </tr> <tr> <td> <pre><code class="language-macaulay2">i3 : L#?12 o3 = false</code></pre> </td> </tr> </table> <div> <p>If <span class="tt">x</span> is a hash table or database, <span class="tt">x#?i</span> tells whether there is a value associated with the key <span class="tt">i</span>.</p> </div> <table class="examples"> <tr> <td> <pre><code class="language-macaulay2">i4 : T = new HashTable from {a => 103, b => 89.4, c => 92};</code></pre> </td> </tr> <tr> <td> <pre><code class="language-macaulay2">i5 : T#?a o5 = true</code></pre> </td> </tr> <tr> <td> <pre><code class="language-macaulay2">i6 : T#?A o6 = false</code></pre> </td> </tr> </table> <div> <p>If <span class="tt">x</span> is a string, <span class="tt">x#?i</span> tells if <span class="tt">x</span> has an <span class="tt">i</span>th character.</p> </div> <table class="examples"> <tr> <td> <pre><code class="language-macaulay2">i7 : s = "a perfectly adequate example of a string";</code></pre> </td> </tr> <tr> <td> <pre><code class="language-macaulay2">i8 : s#?2 o8 = true</code></pre> </td> </tr> <tr> <td> <pre><code class="language-macaulay2">i9 : s#?52 o9 = false</code></pre> </td> </tr> </table> <div> <p><span class="tt">#?</span> can be very useful in avoiding errors from attempting to access nonexistent elements of lists or hash tables.</p> </div> </div> <div> <h2>See also</h2> <ul> <li><span><a title="length or access to elements" href="__sh.html">#</a> -- length or access to elements</span></li> <li><span><a title="length or cardinality of a list, hash table, dictionary, set, or string" href="__sh_sp__List.html"># BasicList</a> -- length or cardinality of a list, hash table, dictionary, set, or string</span></li> <li><span><a title="get element from list" href="___Visible__List_sp_us_sp__Z__Z.html">VisibleList _ ZZ</a> -- get element from list</span></li> <li><span><a href="_hash_sptables.html">hash tables</a></span></li> <li><span><a title="a detailed overview of lists and sequences in Macaulay2" href="_lists_spand_spsequences.html">lists and sequences</a> -- a detailed overview of lists and sequences in Macaulay2</span></li> </ul> </div> <div> <div class="waystouse"> <h2>Ways to use <span class="tt">symbol #?</span>:</h2> <ul> <li><kbd>BasicList #? ZZ</kbd></li> <li><kbd>Database #? String</kbd></li> <li><kbd>Dictionary #? String</kbd></li> <li><kbd>HashTable #? Thing</kbd></li> <li><kbd>List #? ZZ</kbd></li> <li><kbd>Net #? ZZ</kbd></li> <li><kbd>Nothing #? Thing</kbd></li> <li><kbd>Sequence #? ZZ</kbd></li> <li><kbd>String #? ZZ</kbd></li> <li><span><a title="test set membership" href="___Set_sp_sh_qu_sp__Thing.html">Set #? Thing</a> -- test set membership</span></li> </ul> </div> <div class="waystouse"> <h2>For the programmer</h2> <p>The object <a title="check existence of value in a list, hash table, database, or string" href="__sh_qu.html">#?</a> is <span>a <a title="the class of all keywords" href="___Keyword.html">keyword</a></span>.</p> </div> <hr> <div class="waystouse"> <p>The source of this document is in <span class="tt">Macaulay2Doc/ov_hashtables.m2:875:0</span>.</p> </div> </div> </div> </body> </html>