162 lines
5.4 KiB
HTML
162 lines
5.4 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>Hash Table Examples (Guile Reference Manual)</title>
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<meta name="description" content="Hash Table Examples (Guile Reference Manual)">
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<meta name="keywords" content="Hash Table Examples (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="Hash-Tables.html" rel="up" title="Hash Tables">
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<link href="Hash-Table-Reference.html" rel="next" title="Hash Table Reference">
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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="Hash-Table-Examples">
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<div class="nav-panel">
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<p>
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Next: <a href="Hash-Table-Reference.html" accesskey="n" rel="next">Hash Table Reference</a>, Up: <a href="Hash-Tables.html" accesskey="u" rel="up">Hash Tables</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="Hash-Table-Examples-1"><span>6.6.22.1 Hash Table Examples<a class="copiable-link" href="#Hash-Table-Examples-1"> ¶</a></span></h4>
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<p>For demonstration purposes, this section gives a few usage examples of
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some hash table procedures, together with some explanation what they do.
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</p>
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<p>First we start by creating a new hash table with 31 slots, and
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populate it with two key/value pairs.
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(define h (make-hash-table 31))
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;; This is an opaque object
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h
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⇒
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#<hash-table 0/31>
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;; Inserting into a hash table can be done with hashq-set!
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(hashq-set! h 'foo "bar")
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⇒
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"bar"
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(hashq-set! h 'braz "zonk")
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⇒
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"zonk"
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;; Or with hash-create-handle!
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(hashq-create-handle! h 'frob #f)
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⇒
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(frob . #f)
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</pre></div>
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<p>You can get the value for a given key with the procedure
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<code class="code">hashq-ref</code>, but the problem with this procedure is that you
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cannot reliably determine whether a key does exists in the table. The
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reason is that the procedure returns <code class="code">#f</code> if the key is not in
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the table, but it will return the same value if the key is in the
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table and just happens to have the value <code class="code">#f</code>, as you can see in
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the following examples.
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(hashq-ref h 'foo)
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⇒
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"bar"
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(hashq-ref h 'frob)
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⇒
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#f
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(hashq-ref h 'not-there)
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⇒
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#f
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</pre></div>
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<p>It is often better is to use the procedure <code class="code">hashq-get-handle</code>,
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which makes a distinction between the two cases. Just like <code class="code">assq</code>,
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this procedure returns a key/value-pair on success, and <code class="code">#f</code> if the
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key is not found.
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(hashq-get-handle h 'foo)
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⇒
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(foo . "bar")
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(hashq-get-handle h 'not-there)
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⇒
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#f
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</pre></div>
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<p>Interesting results can be computed by using <code class="code">hash-fold</code> to work
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through each element. This example will count the total number of
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elements:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(hash-fold (lambda (key value seed) (+ 1 seed)) 0 h)
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⇒
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3
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</pre></div>
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<p>The same thing can be done with the procedure <code class="code">hash-count</code>, which
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can also count the number of elements matching a particular predicate.
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For example, count the number of elements with string values:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(hash-count (lambda (key value) (string? value)) h)
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⇒
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2
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</pre></div>
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<p>Counting all the elements is a simple task using <code class="code">const</code>:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(hash-count (const #t) h)
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⇒
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3
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</pre></div>
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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="Hash-Table-Reference.html">Hash Table Reference</a>, Up: <a href="Hash-Tables.html">Hash Tables</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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