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# std::pop_heap

From cppreference.com
< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class RandomIt > void pop_heap( RandomIt first, RandomIt last ); | (1) |
template< class RandomIt, class Compare > void pop_heap( RandomIt first, RandomIt last, Compare comp ); | (2) |

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 (largest) element from the heap defined by the range `[first, last)`.

The first version of the function uses operator< to compare the elements, the second uses the given comparison function `comp`.

### [edit] Parameters

first, last | - | the range of elements defining the valid nonempty heap to modify
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); The signature does not need to have const &, but the function object must not modify the objects passed to it. 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 `RandomAccessIterator`.
- The type of dereferenced `RandomIt` must meet the requirements of `MoveAssignable` and `MoveConstructible`.

### [edit] Return value

(none)

### [edit] Complexity

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

### [edit] 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[floor(

    i-1
    2

    )]` does not compare less than `f[i]`.
  - a new element can be added using std::push_heap()
  - the first element can be removed using std::pop_heap()

### [edit] 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

### [edit] See also

push_heap | adds an element to a max heap (function template) [edit]
make_heap | creates a max heap out of a range of elements (function template) [edit]

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