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<h4 class="subsection">10.2.3 Containers</h4>
<p><a name="index-container-1733"></a><a name="index-list-1734"></a><a name="index-vector-1735"></a><a name="index-TREE_005fLIST-1736"></a><a name="index-TREE_005fVEC-1737"></a><a name="index-TREE_005fPURPOSE-1738"></a><a name="index-TREE_005fVALUE-1739"></a><a name="index-TREE_005fVEC_005fLENGTH-1740"></a><a name="index-TREE_005fVEC_005fELT-1741"></a>
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>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.
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