std::ranges::generate_n
From cppreference.com
| Defined in header <algorithm>
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| Call signature |
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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 {};
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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
(C++20) |
saves the result of a function in a range (algorithm function object) |
(C++26) |
fills a range with random numbers from a uniform random bit generator (algorithm function object) |
(C++20) |
assigns a range of elements a certain value (algorithm function object) |
(C++20) |
assigns a value to a number of elements (algorithm function object) |
(C++20) |
applies a function to a range of elements (algorithm function object) |
| assigns the results of successive function calls to N elements in a range (function template) |