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<head><title>ASET2.html -- ACL2 Version 3.1</title></head>
<body text=#000000 bgcolor="#FFFFFF">
<h3>ASET2</h3>set the elements of a 2-dimensional array
<pre>Major Section: <a href="ARRAYS.html">ARRAYS</a>
</pre><p>
<pre>
Example Form:
(aset2 'delta1 a i j 27)
<p>
General Form:
(aset2 name alist i j val)
</pre>
where <code>name</code> is a symbol, <code>alist</code> is a 2-dimensional array named <code>name</code>,
<code>i</code> and <code>j</code> are legal indices into <code>alist</code>, and <code>val</code> is an arbitrary
object. See <a href="ARRAYS.html">arrays</a> for details. Roughly speaking this
function ``modifies'' <code>alist</code> so that the value associated with
<code>(i . j)</code> is <code>val</code>. More precisely, it returns a new array,
<code>alist'</code>, of the same name and dimension as <code>alist</code> that, under
<code><a href="AREF2.html">aref2</a></code>, is everywhere equal to <code>alist</code> except at <code>(i . j)</code> where
the result is <code>val</code>. That is, <code>(aref2 name alist' x y)</code> is
<code>(aref2 name alist x y)</code> for all legal indices <code>x</code> <code>y</code> except
<code>i</code> and <code>j</code> where <code>(aref2 name alist' i j)</code> is <code>val</code>.<p>
In order to ``modify'' <code>alist</code>, <code>aset2</code> <code><a href="CONS.html">cons</a></code>es a new pair onto the
front. If the length of the resulting <code>alist</code> exceeds the
<code>:</code><code><a href="MAXIMUM-LENGTH.html">maximum-length</a></code> entry in the array <a href="HEADER.html">header</a>, <code>aset2</code> compresses the
array as with <code><a href="COMPRESS2.html">compress2</a></code>.<p>
It is generally expected that the ``semantic value'' of <code>name</code> will be
<code>alist</code> (see <a href="ARRAYS.html">arrays</a>). This function operates in virtually
constant time whether this condition is true or not (unless the
<code><a href="COMPRESS2.html">compress2</a></code> operation is required). But the value returned by this
function cannot be used efficiently by subsequent <code>aset2</code> operations
unless <code>alist</code> is the semantic value of <code>name</code> when <code>aset2</code> is executed.
Thus, if the condition is not true, <code>aset2</code> prints a <strong>slow array</strong>
warning to the comment window. See <a href="SLOW-ARRAY-WARNING.html">slow-array-warning</a>.
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