Standard C++ Library
Copyright 1998, Rogue Wave Software, Inc.
NAME
pop_heap
- Moves the largest element off the heap.
SYNOPSIS
template <class RandomAccessIterator>
void
pop_heap(RandomAccessIterator first,
RandomAccessIterator last);
template <class RandomAccessIterator, class Compare>
void
pop_heap(RandomAccessIterator first,
RandomAccessIterator last, Compare comp);
DESCRIPTION
A heap is a particular organization of elements in a range
between two random access iterators [a, b). Its two key pro-
perties are:
1. *a is the largest element in the range.
2. *a may be removed by the pop_heap algorithm or a new
element may be added by the push_heap algorithm, in
O(logN) time.
These properties make heaps useful as priority queues.
The pop_heap algorithm uses the less than (<) operator as
the default comparison. An alternate comparison operator can
be specified.
The pop_heap algorithm can be used as part of an operation
to remove the largest element from a heap. It assumes that
the range [first, last) is a valid heap (in other words,
that first 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 first with the value in the
location last - 1 and makes the range [first, last -1)back
into a heap. You can then access the element in last using
the vector or deque back() member function, or you can
remove the element using the pop_back member function. Note
that pop_heap does not actually remove the element from the
data structure; you must use another function to do that.
COMPLEXITY
pop_heap performs at most 2 * log(last - first) comparisons.
EXAMPLE
//
// heap_ops.cpp
//
#include <algorithm>
#include <vector>
#include <iostream>
using namespace std;
int main(void)
{
int d1[4] = {1,2,3,4};
int d2[4] = {1,3,2,4};
// Set up two vectors
vector<int> v1(d1,d1 + 4), v2(d2,d2 + 4);
// Make heaps
make_heap(v1.begin(),v1.end());
make_heap(v2.begin(),v2.end(),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
ostream_iterator<int,char> out(cout," ");
copy(v1.begin(),v1.end(),out);
cout << endl;
copy(v2.begin(),v2.end(),out);
cout << endl;
// Now let's pop
pop_heap(v1.begin(),v1.end());
pop_heap(v2.begin(),v2.end(),less<int>());
// v1 = (3,x,y,4) and v2 = (3,x,y,4)
// Copy both vectors to cout
copy(v1.begin(),v1.end(),out);
cout << endl;
copy(v2.begin(),v2.end(),out);
cout << endl;
// And push
push_heap(v1.begin(),v1.end());
push_heap(v2.begin(),v2.end(),less<int>());
// v1 = (4,x,y,z) and v2 = (4,x,y,z)
// Copy both vectors to cout
copy(v1.begin(),v1.end(),out);
cout << endl;
copy(v2.begin(),v2.end(),out);
cout << endl;
// Now sort those heaps
sort_heap(v1.begin(),v1.end());
sort_heap(v2.begin(),v2.end(),less<int>());
// v1 = v2 = (1,2,3,4)
// Copy both vectors to cout
copy(v1.begin(),v1.end(),out);
cout << endl;
copy(v2.begin(),v2.end(),out);
cout << endl;
return 0;
}
Program Output
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
WARNINGS
If your compiler does not support default template parame-
ters, you always need to supply the Allocator template argu-
ment. For instance, you need to write:
vector<int, allocator<int> >
instead of:
vector<int>
If your compiler does not support namespaces, then you do
not need the using declaration for std.
SEE ALSO
make_heap, push_heap, sort_heap