make_heap()

Module:  Standard C++ Library   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

Copyright (c) 1994-2006 Rogue Wave Software, a Quovadx Division.
Licensed under the Apache License, Version 2.0.
Contact Rogue Wave about documentation or support issues. You can also seek help from other developers through the Apache stdcxx community (see below).