# std::ranges::pop_heap

< cpp‎ | algorithm‎ | ranges
 Defined in header  Call signature template< std::random_access_iterator I, std::sentinel_for S,           class Comp = ranges::less, class Proj = std::identity > requires std::sortable constexpr I pop_heap( I first, S last, Comp comp = {}, Proj proj = {} ); (1) (since C++20) template< ranges::random_access_range R, class Comp = ranges::less,           class Proj = std::identity > requires std::sortable, Comp, Proj> constexpr ranges::borrowed_iterator_t pop_heap( R&& r, Comp comp = {}, Proj proj = {} ); (2) (since C++20)

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

1) Elements are compared using the given binary comparison function comp and projection object proj.
2) Same as (1), but uses r as the range, as if using as first and ranges::end(r) as last.

In practice, they may be implemented as function objects, or with special compiler extensions.

## Contents

### Parameters

 first, last - the range of elements defining the valid nonempty heap to modify r - the range of elements defining the valid nonempty heap to modify pred - predicate to apply to the projected elements proj - projection to apply to the elements

### Return value

An iterator equal to last.

### Complexity

Given N = ranges::distance(first, last), at most 2log(N) comparisons and 4log(N) projections.

### Notes

The max heap is a range of elements [f, l), arranged with respect to comparator comp and projection proj, that has the following properties:

• With N == l - f, for all 0 < i < N, p == f[(i - 1) / 2], and q == f[i], the expression std::invoke(comp, std::invoke(proj, p), std::invoke(proj, q)) evaluates to false.
• a new element can be added using , in 𝓞(log N) time.
• the first element can be removed using ranges::pop_heap(), in 𝓞(log N) time.

### Example

#include <algorithm>
#include <array>
#include <iostream>
#include <iterator>
#include <string_view>

template <class I = int*>
void print(std::string_view rem, I first = {}, I last = {},
std::string_view term = "\n")
{
for (std::cout << rem; first != last; ++first) {
std::cout << *first << ' ';
}
std::cout << term;
}

int main()
{
std::array v { 3, 1, 4, 1, 5, 9, 2, 6, 5, 3 };
print("initially, v: ", v.cbegin(), v.cend());

std::ranges::make_heap(v);
print("make_heap, v: ", v.cbegin(), v.cend());

print("convert heap into sorted array:");
for (auto n {std::ssize(v)}; n >= 0; --n) {
std::ranges::pop_heap(v.begin(), v.begin() + n);
print("[ ", v.cbegin(), v.cbegin() + n, "]  ");
print("[ ", v.cbegin() + n, v.cend(), "]\n");
}
}

Output:

initially, v: 3 1 4 1 5 9 2 6 5 3
make_heap, v: 9 6 4 5 5 3 2 1 1 3
convert heap into sorted array:
[ 6 5 4 3 5 3 2 1 1 9 ]  [ ]
[ 5 5 4 3 1 3 2 1 6 ]  [ 9 ]
[ 5 3 4 1 1 3 2 5 ]  [ 6 9 ]
[ 4 3 3 1 1 2 5 ]  [ 5 6 9 ]
[ 3 2 3 1 1 4 ]  [ 5 5 6 9 ]
[ 3 2 1 1 3 ]  [ 4 5 5 6 9 ]
[ 2 1 1 3 ]  [ 3 4 5 5 6 9 ]
[ 1 1 2 ]  [ 3 3 4 5 5 6 9 ]
[ 1 1 ]  [ 2 3 3 4 5 5 6 9 ]
[ 1 ]  [ 1 2 3 3 4 5 5 6 9 ]
[ ]  [ 1 1 2 3 3 4 5 5 6 9 ]