make_heap()

Library:  Algorithms

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Algorithm that creates a heap

#include <algorithm>

namespace std {
  template <class RandomAccessIterator>
  void
  make_heap(RandomAccessIterator start,
            RandomAccessIterator finish);

  template <class RandomAccessIterator, class Compare>
  void
  make_heap(RandomAccessIterator start,
            RandomAccessIterator finish, 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:

• *a is the largest element in the range.

• *a may be removed by the pop_heap() algorithm, or a new element can be added by the push_heap() algorithm, in O(log (N)) time.

These properties make heaps useful as priority queues.

The heap algorithms use operator<() as the default comparison. In all of the algorithms, an alternate binary predicate function object can be specified.

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

This algorithm makes at most 3 * (finish - start) comparisons.

//
//  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:

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 

pop_heap(), push_heap(), sort_heap()

ISO/IEC 14882:1998 -- International Standard for Information Systems -- Programming Language C++, 25.3.6.3