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<H1>pop_heap</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>
<tt>Pop_heap</tt> is an overloaded name; there are actually two
<tt>pop_heap</tt> functions.
<pre>
template <class <A href="RandomAccessIterator.html">RandomAccessIterator</A>>
void pop_heap(RandomAccessIterator first, RandomAccessIterator last);
template <class <A href="RandomAccessIterator.html">RandomAccessIterator</A>, class <A href="StrictWeakOrdering.html" tppabs="http://www.sgi.com/Technology/STL/StrictWeakOrdering.shtml">StrictWeakOrdering</A>>
inline void pop_heap(RandomAccessIterator first, RandomAccessIterator last,
StrictWeakOrdering comp);
</pre>
<h3>Description</h3>
<tt>Pop_heap</tt> removes the largest element (that is, <tt>*first</tt>) from the
heap <A href="#1">[1]</A> <tt>[first, last)</tt>.
The two versions of <tt>pop_heap</tt> differ in how they define whether one
element is less than another. The first version compares
objects using <tt>operator<</tt>, and the second compares objects using a
<A href="functors.html">function object</A> <tt>comp</tt>.
<P>
The postcondition for the first version of <tt>pop_heap</tt> is that
<tt><A href="is_heap.html">is_heap</A>(first, last-1)</tt> is <tt>true</tt> and that <tt>*(last - 1)</tt> is the
element that was removed from the heap.
The postcondition for the second version is that
<tt><A href="is_heap.html">is_heap</A>(first, last-1, comp)</tt> is <tt>true</tt> and that <tt>*(last - 1)</tt> is the
element that was removed from the heap. <A href="#2">[2]</A>
<h3>Definition</h3>
Declared in <A href="algo.h">algo.h</A>. The implementation is in <A href="heap.h" tppabs="http://www.sgi.com/Technology/STL/heap.h">heap.h</A>.
<h3>Requirements on types</h3>
For the first version:
<UL>
<LI>
<tt>RandomAccessIterator</tt> is a model of <A href="RandomAccessIterator.html">Random Access Iterator</A>.
<LI>
<tt>RandomAccessIterator</tt> is mutable.
<LI>
<tt>RandomAccessIterator</tt>'s value type is a model of <A href="LessThanComparable.html">LessThan Comparable</A>.
<LI>
The ordering on objects of <tt>RandomAccessIterator</tt>'s value type is a <i>strict
weak ordering</i>, as defined in the <A href="LessThanComparable.html">LessThan Comparable</A> requirements.
</UL>
For the second version:
<UL>
<LI>
<tt>RandomAccessIterator</tt> is a model of <A href="RandomAccessIterator.html">Random Access Iterator</A>.
<LI>
<tt>RandomAccessIterator</tt> is mutable.
<LI>
<tt>StrictWeakOrdering</tt> is a model of <A href="StrictWeakOrdering.html">Strict Weak Ordering</A>.
<LI>
<tt>RandomAccessIterator</tt>'s value type is convertible to
<tt>StrictWeakOrdering</tt>'s argument type.
</UL>
<h3>Preconditions</h3>
For the first version:
<UL>
<LI>
<tt>[first, last)</tt> is a valid range.
<LI>
<tt>[first, last - 1)</tt> is a valid range. That is, <tt>[first, last)</tt> is
nonempty.
<LI>
<tt>[first, last)</tt> is a heap. That is, <tt>is_heap(first, last)</tt> is <tt>true</tt>.
</UL>
For the second version:
<UL>
<LI>
<tt>[first, last)</tt> is a valid range.
<LI>
<tt>[first, last - 1)</tt> is a valid range. That is, <tt>[first, last)</tt> is
nonempty.
<LI>
<tt>[first, last)</tt> is a heap. That is, <tt>is_heap(first, last, comp)</tt> is <tt>true</tt>.
</UL>
<h3>Complexity</h3>
Logarithmic. At most <tt>2 * log(last - first)</tt> comparisons.
<h3>Example</h3>
<pre>
int main()
{
int A[] = {1, 2, 3, 4, 5, 6};
const int N = sizeof(A) / sizeof(int);
make_heap(A, A+N);
cout << "Before pop: ";
copy(A, A+N, ostream_iterator<int>(cout, " "));
pop_heap(A, A+N);
cout << endl << "After pop: ";
copy(A, A+N-1, ostream_iterator<int>(cout, " "));
cout << endl << "A[N-1] = " << A[N-1] << endl;
}
</pre>
<P>
The output is
<pre>
Before pop: 6 5 3 4 2 1
After pop: 5 4 3 1 2
A[N-1] = 6
</pre>
<h3>Notes</h3>
<P><A name="1">[1]</A>
A heap is a particular way of ordering the elements in a range of
<A href="RandomAccessIterator.html">Random Access Iterators</A> <tt>[f, l)</tt>. The reason heaps are useful
(especially for sorting, or as priority queues) is that they satisfy
two important properties. First, <tt>*f</tt> is the largest element in the
heap. Second, it is possible to add an element to a heap (using
<tt><A href="push_heap.html">push_heap</A></tt>), or to remove <tt>*f</tt>, in logarithmic time.
Internally, a heap is a tree represented as a sequential range.
The tree is constructed so that that each
node is less than or equal to its parent node.
<P><A name="2">[2]</A>
<tt>Pop_heap</tt> removes the largest element from a heap, and shrinks the heap.
This means that if you call keep calling <tt>pop_heap</tt> until only
a single element is left in the heap, you will end up with a sorted
range where the heap used to be. This, in fact, is exactly how
<tt><A href="sort_heap.html">sort_heap</A></tt> is implemented.
<h3>See also</h3>
<tt><A href="make_heap.html">make_heap</A></tt>, <tt><A href="push_heap.html" tppabs="http://www.sgi.com/Technology/STL/push_heap.shtml">push_heap</A></tt>, <tt><A href="sort_heap.html" tppabs="http://www.sgi.com/Technology/STL/sort_heap.shtml">sort_heap</A></tt>, <tt><A href="is_heap.html" tppabs="http://www.sgi.com/Technology/STL/is_heap.shtml">is_heap</A></tt>, <tt><A href="sort.html" tppabs="http://www.sgi.com/Technology/STL/sort.shtml">sort</A></tt>
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