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std::ranges::generate_n - cppreference.com

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Defined in header <algorithm>

Call signature

template< std::input_or_output_iterator O, std::copy_constructible F > requires std::invocable<F&> && std::indirectly_writable<O, std::invoke_result_t<F&>> constexpr O generate_n( O first, std::iter_difference_t<O> n, F gen );

 (since C++20)

Assigns the result of successive invocations of the function object gen to each element in the range [first, first + n), if 0 < n. Does nothing otherwise.

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

Parameters

first - the beginning of the range of elements to modify
n - number of elements to modify
gen - the generator function object.

Return value

Iterator one past the last element assigned if 0 < count, first otherwise.

Complexity

Exactly n invocations of gen() and assignments.

Possible implementation

struct generate_n_fn
{
    template<std::input_or_output_iterator O, std::copy_constructible F>
    requires std::invocable<F&> && std::indirectly_writable<O, std::invoke_result_t<F&>>
    constexpr O operator()(O first, std::iter_difference_t<O> n, F gen) const
    {
        for (; n-- > 0; *first = std::invoke(gen), ++first)
        {}
        return first;
    }
};

inline constexpr generate_n_fn generate_n {};

Example

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

auto dice()
{
    static std::uniform_int_distribution<int> distr {1, 6};
    static std::random_device engine;
    static std::mt19937 noise {engine()};
    return distr(noise);
}

void print(const auto& v, std::string_view comment)
{
    for (int i : v)
        std::cout << i << ' ';
    std::cout << '(' << comment << ")\n";
}

int main()
{
    std::array<int, 8> v;

    std::ranges::generate_n(v.begin(), v.size(), dice);
    print(v, "dice");

    std::ranges::generate_n(v.begin(), v.size(), [n {0}] mutable { return n++; });
    // same effect as std::iota(v.begin(), v.end(), 0);
    print(v, "iota");
}

Possible output: 

5 5 2 2 6 6 3 5 (dice)
0 1 2 3 4 5 6 7 (iota)

See also

saves the result of a function in a range
(algorithm function object)[edit]
fills a range with random numbers from a uniform random bit generator
(algorithm function object)[edit]
assigns a range of elements a certain value
(algorithm function object)[edit]
assigns a value to a number of elements
(algorithm function object)[edit]
applies a function to a range of elements
(algorithm function object)[edit]
assigns the results of successive function calls to N elements in a range
(function template) [edit]