NAME¶

std::pop_heap - std::pop_heap

Synopsis¶

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

Swaps the value in the position first and the value in the position last-1 and makes
the subrange [first, last-1) into a heap. This has the effect of removing the first
element from the heap defined by the range [first, last).

The first version of the function uses operator< to compare the elements, which
makes the heap a max heap. The second uses the given comparison function comp.

Parameters¶

first, last - the range of elements defining the valid nonempty heap to modify
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);
comp -
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 ValueSwappable and
LegacyRandomAccessIterator.
-
The type of dereferenced RandomIt must meet the requirements of MoveAssignable and
MoveConstructible.

Return value¶

(none)

Complexity¶

At most 2×log(N) comparisons where N=std::distance(first, last).

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¶

// Run this code

#include <iostream>
#include <algorithm>
#include <vector>

int main()
{
std::vector<int> v { 3, 1, 4, 1, 5, 9 };

std::make_heap(v.begin(), v.end());

std::cout << "v: ";
for (auto i : v) std::cout << i << ' ';
std::cout << '\n';

std::pop_heap(v.begin(), v.end()); // moves the largest to the end

std::cout << "after pop_heap: ";
for (auto i : v) std::cout << i << ' ';
std::cout << '\n';

int largest = v.back();
v.pop_back(); // actually removes the largest element
std::cout << "largest element: " << largest << '\n';

std::cout << "heap without largest: ";
for (auto i : v) std::cout << i << ' ';
std::cout << '\n';
}

Output:¶

v: 9 5 4 1 1 3
after pop_heap: 5 3 4 1 1 9
largest element: 9
heap without largest: 5 3 4 1 1

See also¶

push_heap adds an element to a max heap
(function template)
is_heap checks if the given range is a max heap
(C++11) (function template)
is_heap_until finds the largest subrange that is a max heap
(C++11) (function template)
make_heap creates a max heap out of a range of elements
(function template)
sort_heap turns a max heap into a range of elements sorted in ascending order
(function template)
ranges::pop_heap removes the largest element from a max heap
(C++20) (niebloid)