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<title>Symbol Keys (Guile Reference Manual)</title>
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<link href="Concept-Index.html" rel="index" title="Concept Index">
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
<link href="Symbols.html" rel="up" title="Symbols">
<link href="Symbol-Variables.html" rel="next" title="Symbol Variables">
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<div class="subsubsection-level-extent" id="Symbol-Keys">
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<p>
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> &nbsp; [<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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<hr>
<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"> &para;</a></span></h4>
<p>Given their efficiency and descriptive power, it is natural to use
symbols as the keys in an association list or hash table.
</p>
<p>To illustrate this, consider a more structured representation of the car
properties example from the preceding subsection. Rather than
mixing all the properties up together in a flat list, we could use an
association list like this:
</p>
<div class="example lisp">
<pre class="lisp-preformatted">(define car1-properties '((color . red)
(transmission . manual)
(fuel . unleaded)
(steering . power-assisted)))
</pre></div>
<p>Notice how this structure is more explicit and extensible than the flat
list. For example it makes clear that <code class="code">manual</code> refers to the
transmission rather than, say, the windows or the locking of the car.
It also allows further properties to use the same symbols among their
possible values without becoming ambiguous:
</p>
<div class="example lisp">
<pre class="lisp-preformatted">(define car1-properties '((color . red)
(transmission . manual)
(fuel . unleaded)
(steering . power-assisted)
(seat-color . red)
(locking . manual)))
</pre></div>
<p>With a representation like this, it is easy to use the efficient
<code class="code">assq-XXX</code> family of procedures (see <a class="pxref" href="Association-Lists.html">Association Lists</a>) to
extract or change individual pieces of information:
</p>
<div class="example lisp">
<pre class="lisp-preformatted">(assq-ref car1-properties 'fuel) &rArr; unleaded
(assq-ref car1-properties 'transmission) &rArr; manual
(assq-set! car1-properties 'seat-color 'black)
&rArr;
((color . red)
(transmission . manual)
(fuel . unleaded)
(steering . power-assisted)
(seat-color . black)
(locking . manual)))
</pre></div>
<p>Hash tables also have keys, and exactly the same arguments apply to the
use of symbols in hash tables as in association lists. The hash value
that Guile uses to decide where to add a symbol-keyed entry to a hash
table can be obtained by calling the <code class="code">symbol-hash</code> procedure:
</p>
<dl class="first-deffn">
<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"> &para;</a></span></dt>
<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"> &para;</a></span></dt>
<dd><p>Return a hash value for <var class="var">symbol</var>.
</p></dd></dl>
<p>See <a class="ref" href="Hash-Tables.html">Hash Tables</a> for information about hash tables in general, and
for why you might choose to use a hash table rather than an association
list.
</p>
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<hr>
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<p>
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> &nbsp; [<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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