134 lines
6 KiB
HTML
134 lines
6 KiB
HTML
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<!DOCTYPE html>
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<html>
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<!-- Created by GNU Texinfo 7.1, https://www.gnu.org/software/texinfo/ -->
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<!-- This manual documents Guile version 3.0.10.
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Copyright (C) 1996-1997, 2000-2005, 2009-2023 Free Software Foundation,
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Inc.
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with no
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Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A
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copy of the license is included in the section entitled "GNU Free
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Documentation License." -->
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<title>Symbol Keys (Guile Reference Manual)</title>
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<meta name="description" content="Symbol Keys (Guile Reference Manual)">
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<meta name="keywords" content="Symbol Keys (Guile Reference Manual)">
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<meta name="viewport" content="width=device-width,initial-scale=1">
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<link href="index.html" rel="start" title="Top">
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Symbols.html" rel="up" title="Symbols">
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<link href="Symbol-Variables.html" rel="next" title="Symbol Variables">
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<link href="Symbol-Data.html" rel="prev" title="Symbol Data">
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<style type="text/css">
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<link rel="stylesheet" type="text/css" href="https://www.gnu.org/software/gnulib/manual.css">
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</head>
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<body lang="en">
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<div class="subsubsection-level-extent" id="Symbol-Keys">
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<div class="nav-panel">
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<p>
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Next: <a href="Symbol-Variables.html" accesskey="n" rel="next">Symbols as Denoting Variables</a>, Previous: <a href="Symbol-Data.html" accesskey="p" rel="prev">Symbols as Discrete Data</a>, Up: <a href="Symbols.html" accesskey="u" rel="up">Symbols</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<h4 class="subsubsection" id="Symbols-as-Lookup-Keys"><span>6.6.6.2 Symbols as Lookup Keys<a class="copiable-link" href="#Symbols-as-Lookup-Keys"> ¶</a></span></h4>
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<p>Given their efficiency and descriptive power, it is natural to use
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symbols as the keys in an association list or hash table.
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</p>
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<p>To illustrate this, consider a more structured representation of the car
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properties example from the preceding subsection. Rather than
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mixing all the properties up together in a flat list, we could use an
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association list like this:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(define car1-properties '((color . red)
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(transmission . manual)
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(fuel . unleaded)
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(steering . power-assisted)))
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</pre></div>
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<p>Notice how this structure is more explicit and extensible than the flat
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list. For example it makes clear that <code class="code">manual</code> refers to the
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transmission rather than, say, the windows or the locking of the car.
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It also allows further properties to use the same symbols among their
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possible values without becoming ambiguous:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(define car1-properties '((color . red)
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(transmission . manual)
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(fuel . unleaded)
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(steering . power-assisted)
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(seat-color . red)
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(locking . manual)))
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</pre></div>
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<p>With a representation like this, it is easy to use the efficient
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<code class="code">assq-XXX</code> family of procedures (see <a class="pxref" href="Association-Lists.html">Association Lists</a>) to
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extract or change individual pieces of information:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(assq-ref car1-properties 'fuel) ⇒ unleaded
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(assq-ref car1-properties 'transmission) ⇒ manual
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(assq-set! car1-properties 'seat-color 'black)
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⇒
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((color . red)
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(transmission . manual)
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(fuel . unleaded)
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(steering . power-assisted)
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(seat-color . black)
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(locking . manual)))
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</pre></div>
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<p>Hash tables also have keys, and exactly the same arguments apply to the
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use of symbols in hash tables as in association lists. The hash value
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that Guile uses to decide where to add a symbol-keyed entry to a hash
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table can be obtained by calling the <code class="code">symbol-hash</code> procedure:
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-symbol_002dhash"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">symbol-hash</strong> <var class="def-var-arguments">symbol</var><a class="copiable-link" href="#index-symbol_002dhash"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fsymbol_005fhash"><span class="category-def">C Function: </span><span><strong class="def-name">scm_symbol_hash</strong> <var class="def-var-arguments">(symbol)</var><a class="copiable-link" href="#index-scm_005fsymbol_005fhash"> ¶</a></span></dt>
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<dd><p>Return a hash value for <var class="var">symbol</var>.
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</p></dd></dl>
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<p>See <a class="ref" href="Hash-Tables.html">Hash Tables</a> for information about hash tables in general, and
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for why you might choose to use a hash table rather than an association
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list.
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</p>
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</div>
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<hr>
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<div class="nav-panel">
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<p>
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Next: <a href="Symbol-Variables.html">Symbols as Denoting Variables</a>, Previous: <a href="Symbol-Data.html">Symbols as Discrete Data</a>, Up: <a href="Symbols.html">Symbols</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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