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100 lines
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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<title>Dictionary Types (Guile Reference Manual)</title>
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<div class="subsection-level-extent" id="Dictionary-Types">
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<p>
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Next: <a href="Association-Lists.html" accesskey="n" rel="next">Association Lists</a>, Previous: <a href="Structures.html" accesskey="p" rel="prev">Structures</a>, Up: <a href="Data-Types.html" accesskey="u" rel="up">Data Types</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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<h4 class="subsection" id="Dictionary-Types-1"><span>6.6.19 Dictionary Types<a class="copiable-link" href="#Dictionary-Types-1"> ¶</a></span></h4>
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<p>A <em class="dfn">dictionary</em> object is a data structure used to index
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information in a user-defined way. In standard Scheme, the main
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aggregate data types are lists and vectors. Lists are not really
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indexed at all, and vectors are indexed only by number
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(e.g. <code class="code">(vector-ref foo 5)</code>). Often you will find it useful
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to index your data on some other type; for example, in a library
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catalog you might want to look up a book by the name of its
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author. Dictionaries are used to help you organize information in
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such a way.
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</p>
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<p>An <em class="dfn">association list</em> (or <em class="dfn">alist</em> for short) is a list of
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key-value pairs. Each pair represents a single quantity or
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object; the <code class="code">car</code> of the pair is a key which is used to
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identify the object, and the <code class="code">cdr</code> is the object’s value.
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</p>
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<p>A <em class="dfn">hash table</em> also permits you to index objects with
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arbitrary keys, but in a way that makes looking up any one object
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extremely fast. A well-designed hash system makes hash table
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lookups almost as fast as conventional array or vector references.
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</p>
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<p>Alists are popular among Lisp programmers because they use only
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the language’s primitive operations (lists, <em class="dfn">car</em>, <em class="dfn">cdr</em>
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and the equality primitives). No changes to the language core are
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necessary. Therefore, with Scheme’s built-in list manipulation
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facilities, it is very convenient to handle data stored in an
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association list. Also, alists are highly portable and can be
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easily implemented on even the most minimal Lisp systems.
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</p>
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<p>However, alists are inefficient, especially for storing large
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quantities of data. Because we want Guile to be useful for large
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software systems as well as small ones, Guile provides a rich set
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of tools for using either association lists or hash tables.
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</p>
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Next: <a href="Association-Lists.html">Association Lists</a>, Previous: <a href="Structures.html">Structures</a>, Up: <a href="Data-Types.html">Data Types</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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