# std::ranges::pop_heap

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< cpp‎ | algorithm‎ | ranges

Defined in header `<algorithm>` |  |
Call signature |  |
template< std::random_access_iterator I, std::sentinel_for<I> S, class Comp = ranges::less, class Proj = std::identity > requires std::sortable<I, Comp, Proj> 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<ranges::iterator_t<R>, Comp, Proj> constexpr ranges::borrowed_iterator_t<R> 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 ranges::begin(r) as first and ranges::end(r) as last.

The function-like entities described on this page are niebloids, that is:

- Explicit template argument lists cannot be specified when calling any of them.
- None of them are visible to argument-dependent lookup.
- When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

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

### 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 \(\scriptsize 2\log{(N)}\)2log(N) comparisons and \(\scriptsize 4\log{(N)}\)4log(N) projections.

### Notes

A 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`, `p = f[(i - 1) / 2]`, and `q = f[i]`, for all `0 < i < N`, the expression std::invoke(comp, std::invoke(proj, p), std::invoke(proj, q)) evaluates to false.
  - A new element can be added using ranges::push_heap, in \(\scriptsize \mathcal{O}(\log N)\)𝓞(log N) time.
  - The first element can be removed using ranges::pop_heap, in \(\scriptsize \mathcal{O}(\log N)\)𝓞(log N) time.

### Example

Run this code

    #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 ]

### See also

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

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