make_heap

Algorithm

• Summary
• Data Type and Member Function Indexes
• Synopsis
• Description
• Complexity
• Example
• Warnings
• See Also

Creates a heap.

None

#include <algorithm>
template <class RandomAccessIterator>
  void
  make_heap(RandomAccessIterator first,
            RandomAccessIterator last);

template <class RandomAccessIterator, class Compare>
  void
  make_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 can be added by the push_heap algorithm, in O(logN) time.

These properties make heaps useful as priority queues.

The heap algorithms use less than (operator<) as the default comparison. In all of the algorithms, an alternate comparison operator can be specified.

The first version of the make_heap algorithm arranges the elements in the range [first, last) into a heap using less than (operator<) to perform comparisons. The second version uses the comparison operator comp to perform the comparisons. Since the only requirements for a heap are the two listed above, make_heap is not required to do anything within the range (first, last - 1).

This algorithm makes at most 3 * (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, then you always need to supply the Allocator template argument. For instance, you have 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.

pop_heap, push_heap and sort_heap