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<head><title>COMPRESS1.html -- ACL2 Version 3.1</title></head>
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<h3>COMPRESS1</h3>remove irrelevant pairs from a 1-dimensional array
<pre>Major Section: <a href="ARRAYS.html">ARRAYS</a>
</pre><p>
<pre>
Example Form:
(compress1 'delta1 a)
<p>
General Form:
(compress1 name alist)
</pre>
where <code>name</code> is a symbol and <code>alist</code> is a 1-dimensional array named
<code>name</code>. See <a href="ARRAYS.html">arrays</a> for details. Logically speaking, this
function removes irrelevant pairs from <code>alist</code>, possibly shortening
it. The function returns a new array, <code>alist'</code>, of the same name and
dimension as <code>alist</code>, that, under <code><a href="AREF1.html">aref1</a></code>, is everywhere equal to <code>alist</code>.
That is, <code>(aref1 name alist' i)</code> is <code>(aref1 name alist i)</code>, for all
legal indices <code>i</code>. <code>Alist'</code> may be shorter than <code>alist</code> and the
non-irrelevant pairs may occur in a different order than in <code>alist</code>.<p>
Practically speaking, this function plays an important role in the
efficient implementation of <code><a href="AREF1.html">aref1</a></code>. In addition to creating the new
array, <code>alist'</code>, <code>compress1</code> makes that array the ``semantic value'' of
<code>name</code> and allocates a raw lisp array to <code>name</code>. For each legal index,
<code>i</code>, that raw lisp array contains <code>(aref1 name alist' i)</code> in slot <code>i</code>.
Thus, subsequent <code><a href="AREF1.html">aref1</a></code> operations can be executed in virtually
constant time provided they are given <code>name</code> and the <code>alist'</code> returned
by the most recently executed <code>compress1</code> or <code><a href="ASET1.html">aset1</a></code> on <code>name</code>.
See <a href="ARRAYS.html">arrays</a>.<p>
In general, <code>compress1</code> returns an alist whose <code><a href="CDR.html">cdr</a></code> is an association
list whose keys are nonnegative integers in ascending order. However, if the
<code><a href="HEADER.html">header</a></code> specifies an <code>:order</code> of <code>></code> then the keys will occur in
descending order, and if the <code>:order</code> is <code>:none</code> then the keys will not
be sorted, i.e., <code>compress1</code> is logically the identity function (though it
still attaches an array under the hood).
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