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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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