133 lines
6.9 KiB
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133 lines
6.9 KiB
HTML
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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<title>SRFI-69 Creating hash tables (Guile Reference Manual)</title>
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<div class="subsubsection-level-extent" id="SRFI_002d69-Creating-hash-tables">
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<p>
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Next: <a href="SRFI_002d69-Accessing-table-items.html" accesskey="n" rel="next">Accessing table items</a>, Up: <a href="SRFI_002d69.html" accesskey="u" rel="up">SRFI-69 - Basic 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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<hr>
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<h4 class="subsubsection" id="Creating-hash-tables"><span>7.5.39.1 Creating hash tables<a class="copiable-link" href="#Creating-hash-tables"> ¶</a></span></h4>
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<dl class="first-deffn">
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<dt class="deffn" id="index-make_002dhash_002dtable-1"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">make-hash-table</strong> <var class="def-var-arguments">[equal-proc hash-proc #:weak weakness start-size]</var><a class="copiable-link" href="#index-make_002dhash_002dtable-1"> ¶</a></span></dt>
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<dd><p>Create and answer a new hash table with <var class="var">equal-proc</var> as the
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equality function and <var class="var">hash-proc</var> as the hashing function.
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</p>
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<p>By default, <var class="var">equal-proc</var> is <code class="code">equal?</code>. It can be any
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two-argument procedure, and should answer whether two keys are the
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same for this table’s purposes.
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</p>
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<p>By default <var class="var">hash-proc</var> assumes that <code class="code">equal-proc</code> is no
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coarser than <code class="code">equal?</code> unless it is literally <code class="code">string-ci=?</code>.
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If provided, <var class="var">hash-proc</var> should be a two-argument procedure that
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takes a key and the current table size, and answers a reasonably good
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hash integer between 0 (inclusive) and the size (exclusive).
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</p>
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<p><var class="var">weakness</var> should be <code class="code">#f</code> or a symbol indicating how “weak”
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the hash table is:
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</p>
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<dl class="table">
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<dt><code class="code">#f</code></dt>
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<dd><p>An ordinary non-weak hash table. This is the default.
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</p>
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</dd>
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<dt><code class="code">key</code></dt>
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<dd><p>When the key has no more non-weak references at GC, remove that entry.
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</p>
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</dd>
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<dt><code class="code">value</code></dt>
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<dd><p>When the value has no more non-weak references at GC, remove that
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entry.
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</p>
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</dd>
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<dt><code class="code">key-or-value</code></dt>
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<dd><p>When either has no more non-weak references at GC, remove the
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association.
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</p></dd>
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</dl>
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<p>As a legacy of the time when Guile couldn’t grow hash tables,
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<var class="var">start-size</var> is an optional integer argument that specifies the
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approximate starting size for the hash table, which will be rounded to
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an algorithmically-sounder number.
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</p></dd></dl>
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<p>By <em class="dfn">coarser</em> than <code class="code">equal?</code>, we mean that for all <var class="var">x</var> and
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<var class="var">y</var> values where <code class="code">(<var class="var">equal-proc</var> <var class="var">x</var> <var class="var">y</var>)</code>,
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<code class="code">(equal? <var class="var">x</var> <var class="var">y</var>)</code> as well. If that does not hold for
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your <var class="var">equal-proc</var>, you must provide a <var class="var">hash-proc</var>.
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</p>
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<p>In the case of weak tables, remember that <em class="dfn">references</em> above
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always refers to <code class="code">eq?</code>-wise references. Just because you have a
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reference to some string <code class="code">"foo"</code> doesn’t mean that an association
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with key <code class="code">"foo"</code> in a weak-key table <em class="emph">won’t</em> be collected;
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it only counts as a reference if the two <code class="code">"foo"</code>s are <code class="code">eq?</code>,
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regardless of <var class="var">equal-proc</var>. As such, it is usually only sensible
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to use <code class="code">eq?</code> and <code class="code">hashq</code> as the equivalence and hash
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functions for a weak table. See <a class="xref" href="Weak-References.html">Weak References</a>, for more
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information on Guile’s built-in weak table support.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-alist_002d_003ehash_002dtable-1"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">alist->hash-table</strong> <var class="def-var-arguments">alist [equal-proc hash-proc #:weak weakness start-size]</var><a class="copiable-link" href="#index-alist_002d_003ehash_002dtable-1"> ¶</a></span></dt>
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<dd><p>As with <code class="code">make-hash-table</code>, but initialize it with the
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associations in <var class="var">alist</var>. Where keys are repeated in <var class="var">alist</var>,
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the leftmost association takes precedence.
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</p></dd></dl>
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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="SRFI_002d69-Accessing-table-items.html">Accessing table items</a>, Up: <a href="SRFI_002d69.html">SRFI-69 - Basic 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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</body>
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