std::ranges::pop_heap(3)

std::ranges::pop_heap(3) is the title, and it comes from the C++ Standard Libary manual — yes, “Libary” is exactly how the header spells it, bestie, and we respect the lore. 💖

🎀 NAME 🎀

std::ranges::pop_heap — std::ranges::pop_heap. That’s the name. That’s the whole name. Iconic behavior, honestly. ✨

🌸 Synopsis 🌸

Defined in header <algorithm>. Call signature goes a little like this, and yes, we’re keeping every template detail because she is main character energy:

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>
(1) (since C++20)
constexpr I

    pop_heap( I first, S last, Comp comp = {}, Proj proj = {} );
template< ranges::random_access_range R, class Comp =
ranges::less,

          class Proj = std::identity >
(2) (since C++20)
requires std::sortable<ranges::iterator_t<R>, Comp, Proj>
constexpr ranges::borrowed_iterator_t<R>

    pop_heap( R&& r, Comp comp = {}, Proj proj = {} );

Okay so here’s the vibe: it 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). Slay. 💅

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. No cap. 🫶

💜 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. That’s it, bestie — she returns last and minds her business. 😌

🔥 Complexity 🔥

Given N = ranges::distance(first, last), at most 2log(N) comparisons and 4log(N) projections.

The page literally spells the math as \(\scriptsize 2\log{(N)}\)2log(N) for the comparisons and \(\scriptsize 4\log{(N)}\)4log(N) for the projections, so there you go — every little notation crumb is here. 🍓

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

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This lil page is from http://cppreference.com, dated 2024.06.10 — mwah, kisses to the footer, periodt. 💋✨