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<Title>partition</Title>
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<H1>partition</H1>
<Table CellPadding=0 CellSpacing=0 width=100%>
<TR>
<TD Align=left><Img src = "algorithms.gif" Alt="" WIDTH = "194" HEIGHT = "38" ></TD>
<TD Align=right><Img src = "function.gif" Alt="" WIDTH = "194" HEIGHT = "38" ></TD>
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<TR>
<TD Align=left VAlign=top><b>Category</b>: algorithms</TD>
<TD Align=right VAlign=top><b>Component type</b>: function</TD>
</TR>
</Table>
<h3>Prototype</h3>
<pre>
template <class <A href="ForwardIterator.html">ForwardIterator</A>, class <A href="Predicate.html">Predicate</A>>
ForwardIterator partition(ForwardIterator first,
ForwardIterator last, Predicate pred)
</pre>
<h3>Description</h3>
<tt>Partition</tt> reorders the elements in the range <tt>[first, last)</tt> based
on the <A href="functors.html">function object</A> <tt>pred</tt>, such that
the elements that satisfy <tt>pred</tt> precede the elements
that fail to satisfy it. The postcondition is that, for some
iterator <tt>middle</tt> in the range <tt>[first, last)</tt>,
<tt>pred(*i)</tt> is <tt>true</tt> for every iterator <tt>i</tt> in the range <tt>[first, middle)</tt> and
<tt>false</tt> for every iterator <tt>i</tt> in the range <tt>[middle, last)</tt>. <A href="#1">[1]</A>
The return value of <tt>partition</tt> is <tt>middle</tt>.
<h3>Definition</h3>
Defined in the standard header <A href="algorithm">algorithm</A>, and in the nonstandard
backward-compatibility header <A href="algo.h">algo.h</A>.
<h3>Requirements on types</h3>
<UL>
<LI>
<tt>ForwardIterator</tt> is a model of <A href="ForwardIterator.html">Forward Iterator</A>.
<LI>
<tt>Predicate</tt> is a model of <A href="Predicate.html">Predicate</A>.
<LI>
<tt>ForwardIterator</tt>'s value type is convertible to <tt>Predicate</tt>'s
argument type.
</UL>
<h3>Preconditions</h3>
<UL>
<LI>
<tt>[first, last)</tt> is a valid range.
</UL>
<h3>Complexity</h3>
Linear. Exactly <tt>last - first</tt> applications of <tt>pred</tt>, and at most
<tt>(last - first)/2</tt> swaps.
<h3>Example</h3>
Reorder a sequence so that even numbers precede odd numbers.
<pre>
int A[] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
const int N = sizeof(A)/sizeof(int);
partition(A, A + N,
<A href="unary_compose.html">compose1</A>(<A href="binder2nd.html">bind2nd</A>(<A href="equal_to.html">equal_to</A><int>(), 0),
<A href="binder2nd.html">bind2nd</A>(<A href="modulus.html">modulus</A><int>(), 2)));
<A href="copy.html">copy</A>(A, A + N, <A href="ostream_iterator.html">ostream_iterator</A><int>(cout, " "));
// The output is "10 2 8 4 6 5 7 3 9 1". <A href="#1">[1]</A>
</pre>
<h3>Notes</h3>
<P><A name="1">[1]</A>
The relative order of elements in these two blocks is not necessarily
the same as it was in the original sequence. A different algorithm,
<tt><A href="stable_partition.html">stable_partition</A></tt>, does guarantee to preserve the relative order.
<h3>See also</h3>
<tt><A href="stable_partition.html">stable_partition</A></tt>, <A href="Predicate.html">Predicate</A>, <A href="functors.html">function object</A>
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