# std::ranges::generate_n

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< cpp‎ | algorithm‎ | ranges

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:

- 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.

### 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

Run this code

    #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

ranges::generate (C++20) | saves the result of a function in a range (algorithm function object)
ranges::generate_random (C++26) | fills a range with random numbers from a uniform random bit generator (algorithm function object)
ranges::fill (C++20) | assigns a range of elements a certain value (algorithm function object)
ranges::fill_n (C++20) | assigns a value to a number of elements (algorithm function object)
ranges::transform (C++20) | applies a function to a range of elements (algorithm function object)
generate_n | assigns the results of successive function calls to N elements in a range (function template)

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