std::pop_heap

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< cpplrm; | algorithm
Algorithm library
Execution policies (C++17)
Non-modifying sequence operations
(C++11)(C++11)(C++11)
(C++17)
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
(C++11)
Binary search operations
Set operations (on sorted ranges)
Heap operations
(C++11)
pop_heap
Minimum/maximum operations
(C++11)
(C++17)
Permutations
Numeric operations
C library
Defined in header <algorithm>
(1)
template< class RandomIt >
void pop_heap( RandomIt first, RandomIt last );
(until C++20)
template< class RandomIt >
constexpr void pop_heap( RandomIt first, RandomIt last );
(since C++20)
(2)
template< class RandomIt, class Compare >
void pop_heap( RandomIt first, RandomIt last, Compare comp );
(until C++20)
template< class RandomIt, class Compare >
constexpr void pop_heap( RandomIt first, RandomIt last, Compare comp );
(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 (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.

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.

Return value

(none)

Complexity

At most 2log(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[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()

Example

#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

adds an element to a max heap
(function template)
creates a max heap out of a range of elements
(function template)