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<H2>partition()</H2>
<P><B>Library:</B>&nbsp;&nbsp;<A HREF="2-9.html">Algorithms</A></P>
<PRE><HR><B><I>Function</I></B><HR></PRE>
<UL>
<LI><A HREF="#sec1">Local Index</A></LI>
<LI><A HREF="#sec2">Summary</A></LI>
<LI><A HREF="#sec3">Synopsis</A></LI>
<LI><A HREF="#sec4">Description</A></LI>
<LI><A HREF="#sec5">Complexity</A></LI>
<LI><A HREF="#sec6">Example</A></LI>
<LI><A HREF="#sec7">See Also</A></LI>
<LI><A HREF="#sec8">Standards Conformance</A></LI>
</UL>
<A NAME="sec1"><H3>Local Index</H3></A>
No Entries
<A NAME="sec2"><H3>Summary</H3></A>
<P>Algorithm that places all of the entities that satisfy the given predicate before all of the entities that do not</P>
<A NAME="sec3"><H3>Synopsis</H3></A>
<PRE>#include &lt;algorithm&gt;
namespace std {
template &lt;class BidirectionalIterator, class Predicate&gt;
BidirectionalIterator
partition(BidirectionalIterator start,
BidirectionalIterator finish,
Predicate pred);
}
</PRE>
<A NAME="sec4"><H3>Description</H3></A>
<P>For the range <SAMP>[start, finish)</SAMP>, the <SAMP>partition()</SAMP> algorithm places all elements that satisfy the predicate <SAMP>pred</SAMP> before all elements that do not satisfy <SAMP>pred</SAMP>. It returns an iterator that is one past the end of the group of elements that satisfy <SAMP>pred</SAMP>. In other words, <SAMP>partition()</SAMP> returns <SAMP>i</SAMP> such that for any iterator <SAMP>j</SAMP> in the range <SAMP>[start, i)</SAMP>, <SAMP>pred(*j) == true</SAMP>, and, for any iterator <SAMP>k</SAMP> in the range <SAMP>[i, finish)</SAMP>, <SAMP>pred(*j) == false</SAMP>.</P>
<P>Note that <SAMP>partition()</SAMP> does not necessarily maintain the relative order of the elements that match and elements that do not match the predicate. Use the algorithm <SAMP><A HREF="stable-partition.html">stable_partition()</A></SAMP> if relative order is important.</P>
<A NAME="sec5"><H3>Complexity</H3></A>
<P>The <SAMP>partition()</SAMP> algorithm does at most <SAMP>(finish - start)/2 </SAMP>swaps, and applies the predicate exactly <SAMP>finish - start </SAMP>times. </P>
<A NAME="sec6"><H3>Example</H3></A>
<UL><PRE>//
// prtition.cpp
//
#include &lt;algorithm&gt; // for copy
#include &lt;deque&gt; // for deque
#include &lt;functional&gt; // for unary_function
#include &lt;iostream&gt; // for cout, endl
#include &lt;iterator&gt; // for ostream_iterator
// Create a new predicate from unary_function.
template &lt;class Arg&gt;
struct is_even : public std::unary_function&lt;Arg, bool&gt;
{
bool operator()(const Arg &amp;arg1) const {
return (arg1 % 2) == 0;
}
};
int main ()
{
typedef std::deque&lt;int, std::allocator&lt;int&gt; &gt; Deque;
typedef std::ostream_iterator&lt;int, char,
std::char_traits&lt;char&gt; &gt;
Iter;
// Initialize a deque with an array of integers.
const Deque::value_type a[] = { 1, 2, 3, 4, 5,
6, 7, 8, 9, 10 };
Deque d1 (a + 0, a + sizeof a / sizeof *a);
Deque d2 (d1);
// Print out the original values.
std::cout &lt;&lt; "Unpartitioned values: \t\t";
std::copy (d1.begin (), d1.end (), Iter (std::cout, " "));
// A partition of the deque according to even/oddness.
std::partition (d2.begin (), d2.end (), is_even&lt;int&gt;());
// Output result of partition.
std::cout &lt;&lt; "\nPartitioned values: \t\t";
std::copy (d2.begin (), d2.end (), Iter (std::cout, " "));
// A stable partition of the deque according to
// even/oddness.
std::stable_partition (d1.begin (), d1.end (),
is_even&lt;int&gt;());
// Output result of partition.
std::cout &lt;&lt; "\nStable partitioned values: \t";
std::copy (d1.begin (), d1.end (), Iter (std::cout, " "));
std::cout &lt;&lt; std::endl;
return 0;
}
Program Output:
</PRE></UL>
<UL><PRE>Unpartitioned values: 1 2 3 4 5 6 7 8 9 10
Partitioned values: 10 2 8 4 6 5 7 3 9 1
Stable partitioned values: 2 4 6 8 10 1 3 5 7 9
</PRE></UL>
<A NAME="sec7"><H3>See Also</H3></A>
<P><SAMP><A HREF="stable-partition.html">stable_partition()</A></SAMP></P>
<A NAME="sec8"><H3>Standards Conformance</H3></A>
<P><I>ISO/IEC 14882:1998 -- International Standard for Information Systems -- Programming Language C++, Section 25.2.12</I></P>
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