Category: algorithms | Component type: function |
template <class RandomAccessIterator> void pop_heap(RandomAccessIterator first, RandomAccessIterator last); template <class RandomAccessIterator, class StrictWeakOrdering> inline void pop_heap(RandomAccessIterator first, RandomAccessIterator last, StrictWeakOrdering comp);
The postcondition for the first version of pop_heap is that is_heap(first, last-1) is true and that *(last - 1) is the element that was removed from the heap. The postcondition for the second version is that is_heap(first, last-1, comp) is true and that *(last - 1) is the element that was removed from the heap. [2]
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; }
The output is
Before pop: 6 5 3 4 2 1 After pop: 5 4 3 1 2 A[N-1] = 6
[1] A heap is a particular way of ordering the elements in a range of Random Access Iterators [f, l). The reason heaps are useful (especially for sorting, or as priority queues) is that they satisfy two important properties. First, *f is the largest element in the heap. Second, it is possible to add an element to a heap (using push_heap), or to remove *f, 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.
[2] Pop_heap removes the largest element from a heap, and shrinks the heap. This means that if you call keep calling pop_heap 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 sort_heap is implemented.
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