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<a name="Containers"></a>
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Previous: <a href="Identifiers.html#Identifiers" accesskey="p" rel="prev">Identifiers</a>, Up: <a href="Tree-overview.html#Tree-overview" accesskey="u" rel="up">Tree overview</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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<a name="Containers-1"></a>
<h4 class="subsection">11.2.3 Containers</h4>
<a name="index-container"></a>
<a name="index-list"></a>
<a name="index-vector"></a>
<a name="index-TREE_005fLIST"></a>
<a name="index-TREE_005fVEC"></a>
<a name="index-TREE_005fPURPOSE"></a>
<a name="index-TREE_005fVALUE"></a>
<a name="index-TREE_005fVEC_005fLENGTH"></a>
<a name="index-TREE_005fVEC_005fELT"></a>

<p>Two common container data structures can be represented directly with
tree nodes.  A <code>TREE_LIST</code> is a singly linked list containing two
trees per node.  These are the <code>TREE_PURPOSE</code> and <code>TREE_VALUE</code>
of each node.  (Often, the <code>TREE_PURPOSE</code> contains some kind of
tag, or additional information, while the <code>TREE_VALUE</code> contains the
majority of the payload.  In other cases, the <code>TREE_PURPOSE</code> is
simply <code>NULL_TREE</code>, while in still others both the
<code>TREE_PURPOSE</code> and <code>TREE_VALUE</code> are of equal stature.)  Given
one <code>TREE_LIST</code> node, the next node is found by following the
<code>TREE_CHAIN</code>.  If the <code>TREE_CHAIN</code> is <code>NULL_TREE</code>, then
you have reached the end of the list.
</p>
<p>A <code>TREE_VEC</code> is a simple vector.  The <code>TREE_VEC_LENGTH</code> is an
integer (not a tree) giving the number of nodes in the vector.  The
nodes themselves are accessed using the <code>TREE_VEC_ELT</code> macro, which
takes two arguments.  The first is the <code>TREE_VEC</code> in question; the
second is an integer indicating which element in the vector is desired.
The elements are indexed from zero.
</p>


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