std::ranges::generate_n
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Constrained algorithms
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Defined in header
<algorithm>
Call signature
template< std::input_or_output_iterator O, std::copy_constructible F >
(since C++20)
requires std::invocable <F&> && std::indirectly_writable <O, std::invoke_result_t <F&>>
constexpr O
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.
[edit] Parameters
first
-
the beginning of the range of elements to modify
n
-
number of elements to modify
gen
-
the generator function object.
[edit] Return value
Iterator one past the last element assigned if 0 < count, first otherwise.
[edit] Complexity
Exactly n invocations of gen() and assignments.
[edit] 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 {};
[edit] 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)
[edit] See also
(C++26)
(algorithm function object)[edit]
assigns the results of successive function calls to N elements in a range
(function template) [edit]
(function template) [edit]