std::ranges::shuffle(3)

💖 std::ranges::shuffle(3) 💖

okay bestie, this one is from the C++ Standard Libary manual (yes, that spelling is right there on the page, iykyk) and it’s literally the “make my range go chaotic good” algorithm. ✨

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🌸 NAME 🌸

std::ranges::shuffle — std::ranges::shuffle. That’s the name, and it’s serving exactly that. 💅

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🦋 Synopsis 🦋

Defined in header <algorithm>.

💫 Call signature 💫

Overload (1) is since C++20, and it’s giving very “iterator pair + generator” energy:

template< std::random_access_iterator I, std::sentinel_for<I> S, class Gen >
    requires std::permutable<I> &&
             std::uniform_random_bit_generator<std::remove_reference_t<Gen>>
I shuffle( I first, S last, Gen&& gen );

Overload (2) is also since C++20, and it’s the range-y twin:

template< ranges::random_access_range R, class Gen >
    requires std::permutable<ranges::iterator_t<R>> &&
             std::uniform_random_bit_generator<std::remove_reference_t<Gen>>
ranges::borrowed_iterator_t<R> shuffle( R&& r, Gen&& gen );

So like, overload (1) reorders the elements in the given range [first, last) so that each possible permutation of those elements has equal probability of appearance. Equal probability, no cap. 🎲

Overload (2) is the same as (1), but it uses r as the range, as if using ranges::begin(r) as first and ranges::end(r) as last. Bestie behavior. 🫶

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.

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🌼 Parameters 🌼

• first, last — the range of elements to shuffle randomly
• r — the range of elements to shuffle randomly
• gen — the random number generator

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✨ Return value ✨

An iterator equal to last. That’s it. That’s the return value. 💖

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🔥 Complexity 🔥

Exactly (last - first) - 1 swaps. Not more, not less, periodt. 💀

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🧚 Possible implementation 🧚

struct shuffle_fn
{
    template<std::random_access_iterator I, std::sentinel_for<I> S, class Gen>
    requires std::permutable<I> &&
             std::uniform_random_bit_generator<std::remove_reference_t<Gen>>
    I operator()(I first, S last, Gen&& gen) const
    {
        using diff_t = std::iter_difference_t<I>;
        using distr_t = std::uniform_int_distribution<diff_t>;
        using param_t = typename distr_t::param_type;
        distr_t D;
        const auto n {last - first};
        for (diff_t i {1}; i < n; ++i)
            ranges::iter_swap(first + i, first + D(gen, param_t(0, i)));
        return ranges::next(first, last);
    }

    template<ranges::random_access_range R, class Gen>
    requires std::permutable<ranges::iterator_t<R>> &&
             std::uniform_random_bit_generator<std::remove_reference_t<Gen>>
    ranges::borrowed_iterator_t<R> operator()(R&& r, Gen&& gen) const
    {
        return (*this)(ranges::begin(r), ranges::end(r), std::forward<Gen>(gen));
    }
};

inline constexpr shuffle_fn shuffle {};

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🎀 Example 🎀

// Run this code

#include <algorithm>
#include <array>
#include <iostream>
#include <random>

void print(const auto& a)
{
    for (const auto e : a)
        std::cout << e << ' ';
    std::cout << '\n';
}

int main()
{
    std::array a {'A', 'B', 'C', 'D', 'E', 'F'};
    print(a);

    std::random_device rd;
    std::mt19937 gen {rd()};

    for (int i {}; i != 3; ++i)
    {
        std::ranges::shuffle(a, gen);
        print(a);
    }
}

Possible output:

A B C D E F
F E A C D B
E C B F A D
B A E C F D

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👑 See also 👑

• ranges::next_permutation — generates the next greater lexicographic permutation of a range of elements (C++20) (niebloid)
• ranges::prev_permutation — generates the next smaller lexicographic permutation of a range of elements (C++20) (niebloid)
• random_shuffle — randomly re-orders elements in a range (until C++17) (function template)
• shuffle — randomly re-orders elements in a range (C++11) (function template)

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okay that’s the whole page, bestie 🫶. Cute sign-off time: source is http://cppreference.com, version/date is 2024.06.10, and the page is std::ranges::shuffle(3). Mwah 💋✨