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<h1 id="title-xs"><a href="index.html">The Common Lisp Cookbook</a> &ndash; Miscellaneous</h1>
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<h1 id="title-non-xs"><a href="index.html">The Common Lisp Cookbook</a> &ndash; Miscellaneous</h1>
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<p><a name="opt"></a></p>
<h2 id="re-using-complex-data-structures">Re-using complex data structures</h2>
<p>Sometimes you want your functions to behave in a ‘functional’ way, i.e. return <a href="http://www.lispworks.com/documentation/HyperSpec/Body/26_glo_f.htm#fresh">fresh</a> results without side effects, sometimes you want them to re-use and modify existing data in a destructive way - consider the difference between <a href="http://www.lispworks.com/documentation/HyperSpec/Body/f_append.htm"><code>append</code></a> and <a href="http://www.lispworks.com/documentation/HyperSpec/Body/f_nconc.htm"><code>nconc</code></a> for an example.</p>
<p>Well, you can have your cake and eat it too, by using optional (or keyword) parameters. Here’s an example: Let’s assume you’re writing a function <code>complex-matrix-stuff</code> which takes two matrices <code>m1</code> and <code>m2</code> as its arguments and computes and returns a resulting matrix the size of which depends on <code>m1</code> and <code>m2</code>, i.e. for a fresh result you’ll need an empty matrix which’ll be created by, say, <code>(make-appropriate-result-matrix-for m1 m2)</code>.</p>
<p>The classical textbook way to implement this function will more or less look like this:</p>
<pre><code class="language-lisp">(defun complex-matrix-stuff (m1 m2)
(let ((result (make-appropriate-result-matrix-for m1 m2)))
;; ... compute storing the results in RESULT
result))
</code></pre>
<p>And you’ll use it like this:</p>
<pre><code class="language-lisp">(setq some-matrix (complex-matrix-stuff A B))
</code></pre>
<p>But why not write it like so:</p>
<pre><code class="language-lisp">(defun complex-matrix-stuff (m1 m2
&amp;optional
(result
(make-appropriate-result-matrix-for m1 m2)))
;; ... compute storing the results in RESULT
result)
</code></pre>
<p>Now you have it both ways. You can still “make up results” on the fly as in:</p>
<pre><code class="language-lisp">(setq some-matrix (complex-matrix-stuff A B))
</code></pre>
<p>But you can also (destructively) re-use previously allocated matrices:</p>
<pre><code class="language-lisp">(complex-matrix-stuff A B some-appropriate-matrix-I-built-before)
</code></pre>
<p>Or use your function like this:</p>
<pre><code class="language-lisp">(setq some-other-matrix
(complex-matrix-stuff A B some-appropriate-matrix-I-built-before))
</code></pre>
<p>in which case you’ll end up with:</p>
<pre><code class="language-lisp">* (eq some-other-matrix some-appropriate-matrix-I-built-before)
T
</code></pre>
<p><a name="adjust"></a></p>
<h2 id="using-adjust-array-instead-of-consing-up-new-sequences-with-subseq">Using <code>ADJUST-ARRAY</code> instead of consing up new sequences with <code>SUBSEQ</code></h2>
<p>Most CL functions operating on sequences will accept <code>start</code> and <code>end</code> keywords so you can make them operate on a sub-sequence without actually creating it, i.e. instead of</p>
<pre><code class="language-lisp">(count #\a (subseq long-string from to))
</code></pre>
<p>you should of course use</p>
<pre><code class="language-lisp">(count #\a long-string :start from :end to)
</code></pre>
<p>which’ll yield the same result but not create an unnecessary intermediate sub-sequence.</p>
<p>However, sometimes it looks like you can’t avoid creating new data. Consider a hash table the keys of which are strings. If the key you’re looking for is a sub-string of another string you’ll most likely end up with</p>
<pre><code class="language-lisp">(gethash (subseq original-string from to)
hash-table)
</code></pre>
<p>But you don’t have to. You can create <em>one</em> <a href="http://www.lispworks.com/documentation/HyperSpec/Body/26_glo_d.htm#displaced_array">displaced</a> string and reuse it multiple times with <a href="http://www.lispworks.com/documentation/HyperSpec/Body/f_adjust.htm"><code>adjust-array</code></a>:</p>
<pre><code class="language-lisp">(let ((substring (make-array 0
:element-type 'character
:displaced-to ""
:displaced-index-offset 0)))
;; more code
(gethash
(adjust-array substring (- to from)
:displaced-to original-string
:displaced-index-offset from)
hash-table)
;; even more code
)
</code></pre>
<p class="page-source">
Page source: <a href="https://github.com/LispCookbook/cl-cookbook/blob/master/misc.md">misc.md</a>
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