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| <TITLE>pop_heap()</TITLE> |
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| <H2>pop_heap()</H2> |
| <P><B>Library:</B> <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 <algorithm> |
| |
| namespace std { |
| template <class RandomAccessIterator> |
| void |
| pop_heap(RandomAccessIterator start, |
| RandomAccessIterator finish); |
| |
| template <class RandomAccessIterator, class Compare> |
| 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<()</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 <algorithm> // for copy, make_heap, pop_heap, |
| // and push_heap |
| #include <functional> // for less |
| #include <iostream> // for cout |
| #include <iterator> // for ostream_iterator |
| #include <vector> // for vector |
| |
| |
| |
| template <class charT, class Traits, class T, class Allocator> |
| void print_vector (std::basic_ostream<charT, Traits> &strm, |
| const std::vector<T, Allocator> &v) |
| { |
| std::copy (v.begin (), v.end (), |
| std::ostream_iterator<T, charT, Traits> |
| (strm, " ")); |
| |
| strm << std::endl; |
| } |
| |
| |
| int main () |
| { |
| typedef std::vector<int, std::allocator<int> > 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<int>()); |
| |
| // 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<int>()); |
| |
| 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<int>()); |
| |
| 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<int>()); |
| |
| 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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