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135 lines
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<title>Removing Alist Entries (Guile Reference Manual)</title>
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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="Association-Lists.html" rel="up" title="Association Lists">
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<link href="Sloppy-Alist-Functions.html" rel="next" title="Sloppy Alist Functions">
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<div class="subsubsection-level-extent" id="Removing-Alist-Entries">
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<p>
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Next: <a href="Sloppy-Alist-Functions.html" accesskey="n" rel="next">Sloppy Alist Functions</a>, Previous: <a href="Retrieving-Alist-Entries.html" accesskey="p" rel="prev">Retrieving Alist Entries</a>, Up: <a href="Association-Lists.html" accesskey="u" rel="up">Association Lists</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="subsubsection" id="Removing-Alist-Entries-1"><span>6.6.20.4 Removing Alist Entries<a class="copiable-link" href="#Removing-Alist-Entries-1"> ¶</a></span></h4>
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<p>To remove the element from an association list whose key matches a
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specified key, use <code class="code">assq-remove!</code>, <code class="code">assv-remove!</code> or
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<code class="code">assoc-remove!</code> (depending, as usual, on the level of equality
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required between the key that you specify and the keys in the
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association list).
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</p>
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<p>As with <code class="code">assq-set!</code> and friends, the specified alist may or may not
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be modified destructively, and the only safe way to update a variable
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containing the alist is to <code class="code">set!</code> it to the value that
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<code class="code">assq-remove!</code> and friends return.
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</p>
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<div class="example">
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<pre class="example-preformatted">address-list
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⇒
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(("bob" . "11 Newington Avenue") ("mary" . "34 Elm Road")
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("james" . "1a London Road"))
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(set! address-list (assoc-remove! address-list "mary"))
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address-list
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⇒
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(("bob" . "11 Newington Avenue") ("james" . "1a London Road"))
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</pre></div>
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<p>Note that, when <code class="code">assq/v/oc-remove!</code> is used to modify an
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association list that has been constructed only using the corresponding
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<code class="code">assq/v/oc-set!</code>, there can be at most one matching entry in the
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alist, so the question of multiple entries being removed in one go does
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not arise. If <code class="code">assq/v/oc-remove!</code> is applied to an association
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list that has been constructed using <code class="code">acons</code>, or an
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<code class="code">assq/v/oc-set!</code> with a different level of equality, or any mixture
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of these, it removes only the first matching entry from the alist, even
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if the alist might contain further matching entries. For example:
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</p>
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<div class="example">
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<pre class="example-preformatted">(define address-list '())
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(set! address-list (assq-set! address-list "mary" "11 Elm Street"))
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(set! address-list (assq-set! address-list "mary" "57 Pine Drive"))
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address-list
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⇒
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(("mary" . "57 Pine Drive") ("mary" . "11 Elm Street"))
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(set! address-list (assoc-remove! address-list "mary"))
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address-list
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⇒
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(("mary" . "11 Elm Street"))
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</pre></div>
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<p>In this example, the two instances of the string "mary" are not the same
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when compared using <code class="code">eq?</code>, so the two <code class="code">assq-set!</code> calls add
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two distinct entries to <code class="code">address-list</code>. When compared using
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<code class="code">equal?</code>, both "mary"s in <code class="code">address-list</code> are the same as the
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"mary" in the <code class="code">assoc-remove!</code> call, but <code class="code">assoc-remove!</code> stops
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after removing the first matching entry that it finds, and so one of the
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"mary" entries is left in place.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-assq_002dremove_0021"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">assq-remove!</strong> <var class="def-var-arguments">alist key</var><a class="copiable-link" href="#index-assq_002dremove_0021"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-assv_002dremove_0021"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">assv-remove!</strong> <var class="def-var-arguments">alist key</var><a class="copiable-link" href="#index-assv_002dremove_0021"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-assoc_002dremove_0021"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">assoc-remove!</strong> <var class="def-var-arguments">alist key</var><a class="copiable-link" href="#index-assoc_002dremove_0021"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fassq_005fremove_005fx"><span class="category-def">C Function: </span><span><strong class="def-name">scm_assq_remove_x</strong> <var class="def-var-arguments">(alist, key)</var><a class="copiable-link" href="#index-scm_005fassq_005fremove_005fx"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fassv_005fremove_005fx"><span class="category-def">C Function: </span><span><strong class="def-name">scm_assv_remove_x</strong> <var class="def-var-arguments">(alist, key)</var><a class="copiable-link" href="#index-scm_005fassv_005fremove_005fx"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fassoc_005fremove_005fx"><span class="category-def">C Function: </span><span><strong class="def-name">scm_assoc_remove_x</strong> <var class="def-var-arguments">(alist, key)</var><a class="copiable-link" href="#index-scm_005fassoc_005fremove_005fx"> ¶</a></span></dt>
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<dd><p>Delete the first entry in <var class="var">alist</var> associated with <var class="var">key</var>, and return
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the resulting alist.
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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="Sloppy-Alist-Functions.html">Sloppy Alist Functions</a>, Previous: <a href="Retrieving-Alist-Entries.html">Retrieving Alist Entries</a>, Up: <a href="Association-Lists.html">Association Lists</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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