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std::make_heap

From cppreference.com
< cpp‎ | algorithm
 
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
(C++11)(C++11)(C++11)
(C++17)
Modifying sequence operations
Partitioning operations
Sorting operations
(C++11)
Binary search operations
Set operations (on sorted ranges)
Heap operations
(C++11)
make_heap
Minimum/maximum operations
(C++11)
(C++17)

Permutations
Numeric operations
Operations on uninitialized storage
(C++17)
(C++17)
(C++17)
C library
 
Defined in header <algorithm>
(1)
template< class RandomIt >
void make_heap( RandomIt first, RandomIt last );
(until C++20)
template< class RandomIt >
constexpr void make_heap( RandomIt first, RandomIt last );
(since C++20)
(2)
template< class RandomIt, class Compare >
void make_heap( RandomIt first, RandomIt last, Compare comp );
(until C++20)
template< class RandomIt, class Compare >
constexpr void make_heap( RandomIt first, RandomIt last, Compare comp );
(since C++20)

Constructs a heap in the range [firstlast).

1) The constructed heap is with respect to operator<.
2) The constructed heap is with respect to comp.

Contents

[edit] Parameters

first, last - the range to make the heap from
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);

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 LegacyRandomAccessIterator.
-
The type of dereferenced RandomIt must meet the requirements of MoveAssignable and MoveConstructible.
-
Compare must meet the requirements of Compare.

[edit] Return value

(none)

[edit] Complexity

Given N as std::distance(first, last):

1) At most 3N comparisons using operator<.
2) At most 3N applications of the comparison function comp.

[edit] Notes

A heap with respect to comp (max heap) is a random access range [firstlast) that has the following properties:

  • Given N as last - first, for all integer i where 0 < i < N, bool(comp(first[(i - 1) / 2], first[i])) is false.
  • A new element can be added using std::push_heap, in 𝓞(log N) time.
  • *first can be removed using std::pop_heap, in 𝓞(log N) time.

[edit] Example

#include <algorithm>
#include <functional>
#include <iostream>
#include <string_view>
#include <vector>
 
void print(std::string_view text, std::vector<int> const& v = {})
{
    std::cout << text << ": ";
    for (const auto& e : v)
        std::cout << e << ' ';
    std::cout << '\n';
}
 
int main()
{
    print("Max heap");
 
    std::vector<int> v{3, 2, 4, 1, 5, 9};
    print("initially, v", v);
 
    std::make_heap(v.begin(), v.end());
    print("after make_heap, v", v);
 
    std::pop_heap(v.begin(), v.end());
    print("after pop_heap, v", v);
 
    auto top = v.back();
    v.pop_back();
    print("former top element", {top});
    print("after removing the former top element, v", v);
 
    print("\nMin heap");
 
    std::vector<int> v1{3, 2, 4, 1, 5, 9};
    print("initially, v1", v1);
 
    std::make_heap(v1.begin(), v1.end(), std::greater<>{});
    print("after make_heap, v1", v1);
 
    std::pop_heap(v1.begin(), v1.end(), std::greater<>{});
    print("after pop_heap, v1", v1);
 
    auto top1 = v1.back();
    v1.pop_back();
    print("former top element", {top1});
    print("after removing the former top element, v1", v1);
}

Output:

Max heap:
initially, v: 3 2 4 1 5 9
after make_heap, v: 9 5 4 1 2 3
after pop_heap, v: 5 3 4 1 2 9
former top element: 9
after removing the former top element, v: 5 3 4 1 2
 
Min heap:
initially, v1: 3 2 4 1 5 9
after make_heap, v1: 1 2 4 3 5 9
after pop_heap, v1: 2 3 4 9 5 1
former top element: 1
after removing the former top element, v1: 2 3 4 9 5

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 193 C++98 heap required *first to be the largest element there can be elements equal to *first
LWG 2166 C++98 the heap requirement did not match the
definition of max heap closely enough
requirement improved

[edit] See also

(C++11)
checks if the given range is a max heap
(function template) [edit]
finds the largest subrange that is a max heap
(function template) [edit]
adds an element to a max heap
(function template) [edit]
removes the largest element from a max heap
(function template) [edit]
turns a max heap into a range of elements sorted in ascending order
(function template) [edit]
adapts a container to provide priority queue
(class template) [edit]
creates a max heap out of a range of elements
(niebloid) [edit]