pop_heap

Algorithms

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Moves the largest element off the heap.

None

template <class RandomAccessIterator>
  void
  pop_heap(RandomAccessIterator first,
           RandomAccessIterator last);

template <class RandomAccessIterator, class Compare>
  void
  pop_heap(RandomAccessIterator first,
           RandomAccessIterator last, Compare comp);

A heap is a particular organization of elements in a range between two random access iterators [a, b). Its two key properties 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.

pop_heap performs at most 2 * log(last - first) comparisons.

//
// 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

If your compiler does not support default template parameters, you always need to supply the Allocator template argument. 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.

make_heap, push_heap, sort_heap