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<H2>pop_heap()</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 moves the largest element off the heap</P>
<A NAME="sec3"><H3>Synopsis</H3></A>

<PRE>#include &lt;algorithm&gt;

namespace std {
template &lt;class RandomAccessIterator&gt;
  void
  pop_heap(RandomAccessIterator start,
           RandomAccessIterator finish);

template &lt;class RandomAccessIterator, class Compare&gt;
  void
  pop_heap(RandomAccessIterator start,
           RandomAccessIterator finish, Compare comp);
}
</PRE>
<A NAME="sec4"><H3>Description</H3></A>
<P>A heap is a particular organization of elements in a range between two random access iterators <SAMP>[a, b)</SAMP>. Its two key properties are:</P>
<OL>
<LI><P CLASS="LIST"><SAMP>*a</SAMP> is the largest element in the range.</P></LI>
<LI><P CLASS="LIST"><SAMP>*a</SAMP> may be removed by the <SAMP>pop_heap()</SAMP> algorithm or a new element may be added by the <SAMP>pop_heap()</SAMP> algorithm, in <SAMP>O(log(N))</SAMP> time.</P></LI>
</OL>
<P>These properties make heaps useful as priority queues.</P>
<P>The <SAMP>pop_heap()</SAMP> algorithm uses <SAMP>operator&lt;()</SAMP> as the default comparison. An alternate comparison operator can be specified to the second overload of the function template.</P>
<P>The <SAMP>pop_heap()</SAMP> algorithm can be used as part of an operation to remove the largest element from a heap. It assumes that the range <SAMP>[start, finish)</SAMP> is a valid heap (in other words, that <SAMP>start</SAMP> is the largest element in the heap or the first element based on the alternate comparison operator). It then swaps the value in the location <SAMP>start</SAMP> with the value in the location <SAMP>finish - 1</SAMP> and makes the range <SAMP>[start, finish - 1)</SAMP> back into a heap.</P>
<A NAME="sec5"><H3>Complexity</H3></A>
<P><SAMP>pop_heap()</SAMP> performs at most <SAMP>2 * log(finish - start)</SAMP> comparisons.</P>
<A NAME="sec6"><H3>Example</H3></A>

<UL><PRE>//
//  heap_ops.cpp
//

#include &lt;algorithm&gt;    // for copy, make_heap, pop_heap, 
                        // and push_heap
#include &lt;functional&gt;   // for less
#include &lt;iostream&gt;     // for cout
#include &lt;iterator&gt;     // for ostream_iterator
#include &lt;vector&gt;       // for vector


template &lt;class charT, class Traits, class T, class Allocator&gt;
void print_vector (std::basic_ostream&lt;charT, Traits&gt; &amp;strm,
                   const std::vector&lt;T, Allocator&gt;   &amp;v)
{
    std::copy (v.begin (), v.end (),
               std::ostream_iterator&lt;T, charT, Traits&gt; 
                                    (strm, " "));

    strm &lt;&lt; std::endl;
}


int main ()
{
    typedef std::vector&lt;int, std::allocator&lt;int&gt; &gt; Vector;

    const Vector::value_type d1[] = { 1, 2, 3, 4 };
    const Vector::value_type d2[] = { 1, 3, 2, 4 };

    // Set up two vectors.
    Vector v1 (d1 + 0, d1 + sizeof d1 / sizeof *d1);
    Vector v2 (d2 + 0, d2 + sizeof d2 / sizeof *d2);

    // Make heaps.
    std::make_heap (v1.begin (), v1.end ());
    std::make_heap (v2.begin (), v2.end (), std::less&lt;int&gt;());

    // v1 = (4, x, y, z)  and  v2 = (4, x, y, z)

    // Note that x, y and z represent the remaining values 
    // in the container (other than 4).  The definition of 
    // the heap and heap operations does not require any 
    // particular ordering of these values.

    // Copy both vectors to cout.
    print_vector (std::cout, v1);
    print_vector (std::cout, v2);

    // Now let's pop.
    std::pop_heap (v1.begin (), v1.end ());
    std::pop_heap (v2.begin (), v2.end (), std::less&lt;int&gt;());

    print_vector (std::cout, v1);
    print_vector (std::cout, v2);

    // And push.
    std::push_heap (v1.begin (), v1.end ());
    std::push_heap (v2.begin (), v2.end (), std::less&lt;int&gt;());

    print_vector (std::cout, v1);
    print_vector (std::cout, v2);

    // Now sort those heaps.
    std::sort_heap (v1.begin (), v1.end ());
    std::sort_heap (v2.begin (), v2.end (), std::less&lt;int&gt;());

    print_vector (std::cout, v1);
    print_vector (std::cout, v2);

    return 0;
}


Program Output:
</PRE></UL>
<UL><PRE>4 2 3 1 
4 3 2 1 
3 2 1 4 
3 1 2 4 
4 3 1 2 
4 3 2 1 
1 2 3 4 
1 2 3 4 
</PRE></UL>
<A NAME="sec7"><H3>See Also</H3></A>
<P><SAMP><A HREF="make-heap.html">make_heap()</A></SAMP>,  <SAMP><A HREF="push-heap.html">push_heap()</A></SAMP>,  <SAMP><A HREF="sort-heap.html">sort_heap()</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 25.3.6.2</I></P>

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