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 make_heap()
Apache C++ Standard Library Reference Guide

make_heap()

Library:  Algorithms

Function

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Summary

Algorithm that creates a heap

Synopsis

#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);
}

Description

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).

Complexity

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

Example

//
//  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