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std::ranges::is_heap - cppreference.com

Defined in header <algorithm>

Call signature

template< std::random_access_iterator I, std::sentinel_for<I> S, class Proj = std::identity, std::indirect_strict_weak_order <std::projected<I, Proj>> Comp = ranges::less > constexpr bool is_heap( I first, S last, Comp comp = {}, Proj proj = {} );

 (1) (since C++20)

template< ranges::random_access_range R, class Proj = std::identity, std::indirect_strict_weak_order <std::projected <ranges::iterator_t<R>, Proj>> Comp = ranges::less > constexpr bool is_heap( R&& r, Comp comp = {}, Proj proj = {} );

 (2) (since C++20)

Checks whether the specified range represents a heap with respect to comp and proj.

1) The specified range is [first, last).

2) The specified range is r.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

Parameters

first, last - the iterator-sentinel pair defining the range of elements to examine
r - the range of elements to examine
comp - comparator to apply to the projected elements
proj - projection to apply to the elements

Return value

1) ranges::is_heap_until(first, last, comp, proj) == last

2) ranges::is_heap_until(r, comp, proj) == ranges::end(r)

Complexity

O(N) applications of comp and proj, where N is:

1) ranges::distance(first, last)

2) ranges::distance(r)

Possible implementation

struct is_heap_fn
{
    template<std::random_access_iterator I, std::sentinel_for<I> S,
             class Proj = std::identity,
             std::indirect_strict_weak_order
                 <std::projected<I, Proj>> Comp = ranges::less>
    constexpr bool operator()(I first, S last, Comp comp = {}, Proj proj = {}) const
    {
        return (last == ranges::is_heap_until(first, last,
                                              std::move(comp), std::move(proj)));
    }
    
    template<ranges::random_access_range R, class Proj = std::identity,
             std::indirect_strict_weak_order
                 <std::projected<ranges::iterator_t<R>, Proj>> Comp = ranges::less>
    constexpr bool operator()(R&& r, Comp comp = {}, Proj proj = {}) const
    {
        return (*this)(ranges::begin(r), ranges::end(r),
                       std::move(comp), std::move(proj));
    }
};

inline constexpr is_heap_fn is_heap{};

Example

#include <algorithm>
#include <bit>
#include <cmath>
#include <iostream>
#include <vector>

void out(const auto& what, int n = 1)
{
    while (n-- > 0)
        std::cout << what;
}

void draw_heap(const auto& v)
{
    auto bails = [](int n, int w)
    {
        auto b = [](int w) { out("┌"), out("─", w), out("┴"), out("─", w), out("┐"); };
        n /= 2;
        if (!n)
            return;
        for (out(' ', w); n-- > 0;)
            b(w), out(' ', w + w + 1);
        out('\n');
    };
    
    auto data = [](int n, int w, auto& first, auto last)
    {
        for (out(' ', w); n-- > 0 && first != last; ++first)
            out(*first), out(' ', w + w + 1);
        out('\n');
    };
    
    auto tier = [&](int t, int m, auto& first, auto last)
    {
        const int n{1 << t};
        const int w{(1 << (m - t - 1)) - 1};
        bails(n, w), data(n, w, first, last);
    };
    
    const int m{static_cast<int>(std::ceil(std::log2(1 + v.size())))};
    auto first{v.cbegin()};
    for (int i{}; i != m; ++i)
        tier(i, m, first, v.cend());
}

int main()
{
    std::vector<int> v{3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5, 8, 9, 7, 9, 3, 2, 3, 8};
    
    out("initially, v:\n");
    for (auto i : v)
        std::cout << i << ' ';
    out('\n');
    
    if (!std::ranges::is_heap(v))
    {
        out("making heap...\n");
        std::ranges::make_heap(v);
    }
    
    out("after make_heap, v:\n");
    for (auto t{1U}; auto i : v)
        std::cout << i << (std::has_single_bit(++t) ? " │ " : " ");
    
    out("\n" "corresponding binary tree is:\n");
    draw_heap(v);
}

Output: 

initially, v:
3 1 4 1 5 9 2 6 5 3 5 8 9 7 9 3 2 3 8
making heap...
after make_heap, v:
9 │ 8 9 │ 6 5 8 9 │ 3 5 3 5 3 4 7 2 │ 1 2 3 1
corresponding binary tree is:
               9
       ┌───────┴───────┐
       8               9
   ┌───┴───┐       ┌───┴───┐
   6       5       8       9
 ┌─┴─┐   ┌─┴─┐   ┌─┴─┐   ┌─┴─┐
 3   5   3   5   3   4   7   2
┌┴┐ ┌┴┐ ┌┴┐ ┌┴┐ ┌┴┐ ┌┴┐ ┌┴┐ ┌┴┐
1 2 3 1

See also

finds the largest subrange that is a max heap
(algorithm function object)[edit]
creates a max heap out of a range of elements
(algorithm function object)[edit]
adds an element to a max heap
(algorithm function object)[edit]
removes the largest element from a max heap
(algorithm function object)[edit]
turns a max heap into a range of elements sorted in ascending order
(algorithm function object)[edit]

(C++11)

 checks if the given range is a max heap
(function template) [edit]