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std::make_heap

Defined in header `<algorithm>`
	(1)
template< class RandomIt > void make_heap( RandomIt first, RandomIt last );		(until C++20)
template< class RandomIt > constexpr void make_heap( RandomIt first, RandomIt last );		(since C++20)
	(2)
template< class RandomIt, class Compare > void make_heap( RandomIt first, RandomIt last, Compare comp );		(until C++20)
template< class RandomIt, class Compare > constexpr void make_heap( RandomIt first, RandomIt last, Compare comp );		(since C++20)

Constructs a max heap in the range [first, last). The first version of the function uses operator< to compare the elements, the second uses the given comparison function comp.

Parameters

first, last	-	the range of elements to make the heap from
comp	-	comparison function object (i.e. an object that satisfies the requirements of Compare) which returns `true` if the first argument is less than the second. The signature of the comparison function should be equivalent to the following: `bool cmp(const Type1 &a, const Type2 &b);` While the signature does not need to have `const &`, the function must not modify the objects passed to it and must be able to accept all values of type (possibly const) `Type1` and `Type2` regardless of value category (thus, `Type1 &` is not allowed, nor is `Type1` unless for `Type1` a move is equivalent to a copy (since C++11)). The types Type1 and Type2 must be such that an object of type RandomIt can be dereferenced and then implicitly converted to both of them.
Type requirements
-`RandomIt` must meet the requirements of LegacyRandomAccessIterator.
-The type of dereferenced `RandomIt` must meet the requirements of MoveAssignable and MoveConstructible.

Return value

(none).

Complexity

At most 3 * std::distance(first, last) comparisons.

Notes

A max heap is a range of elements [f, l) that has the following properties:

With N = l - f, for all 0 < i < N, f[(i - 1) / 2] does not compare less than f[i].
A new element can be added using std::push_heap, in \(\scriptsize \mathcal{O}(\log N)\)𝓞(log N) time.
The first element can be removed using std::pop_heap, in \(\scriptsize \mathcal{O}(\log N)\)𝓞(log N) time.

Example

#include <algorithm>
#include <functional>
#include <iostream>
#include <string_view>
#include <vector>
 
void print(std::string_view text, std::vector<int> const& v = {})
{
    std::cout << text << ": ";
    for (const auto& e : v) std::cout << e << ' ';
    std::cout << '\n';
}
 
int main()
{
    print("Max heap");
 
    std::vector<int> v {3, 2, 4, 1, 5, 9};
    print("initially, v", v);
 
    std::make_heap(v.begin(), v.end());
    print("after make_heap, v", v);
 
    std::pop_heap(v.begin(), v.end());
    print("after pop_heap, v", v);
 
    auto top = v.back();
    v.pop_back();
    print("former top element", {top});
    print("after removing the former top element, v", v);
 
    print("\nMin heap");
 
    std::vector<int> v1 {3, 2, 4, 1, 5, 9};
    print("initially, v1", v1);
 
    std::make_heap(v1.begin(), v1.end(), std::greater<>{});
    print("after make_heap, v1", v1);
 
    std::pop_heap(v1.begin(), v1.end(), std::greater<>{});
    print("after pop_heap, v1", v1);
 
    auto top1 = v1.back();
    v1.pop_back();
    print("former top element", {top1});
    print("after removing the former top element, v1", v1);
}

Output:

Max heap:
initially, v: 3 2 4 1 5 9
after make_heap, v: 9 5 4 1 2 3
after pop_heap, v: 5 3 4 1 2 9
former top element: 9
after removing the former top element, v: 5 3 4 1 2
 
Min heap:
initially, v1: 3 2 4 1 5 9
after make_heap, v1: 1 2 4 3 5 9
after pop_heap, v1: 2 3 4 9 5 1
former top element: 1
after removing the former top element, v1: 2 3 4 9 5