std::ranges::for_each_n, std::ranges::for_each_n_result
From cppreference.com
| Defined in header <algorithm>
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| Call signature |
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| template< std::input_iterator I, class Proj = identity, std::indirectly_unary_invocable<std::projected<I, Proj>> Fun > |
(1) | (since C++20) |
| Helper types |
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| template< class I, class F > using for_each_n_result = ranges::in_fun_result<I, F>; |
(2) | (since C++20) |
1) Applies the given function object
f to the projected result by proj of dereferencing each iterator in the range [first, first + n), in order.If the iterator type is mutable, f may modify the elements of the range through the dereferenced iterator. If f returns a result, the result is ignored. If n is less than zero, the behavior is undefined.
The function-like entities described on this page are niebloids, that is:
- Explicit template argument lists may not be specified when calling any of them.
- None of them is visible to argument-dependent lookup.
- When one of them is found by normal unqualified lookup for the name to the left of the function-call operator, it inhibits argument-dependent lookup.
In practice, they may be implemented as function objects, or with special compiler extensions.
Parameters
| first | - | iterator denoting the begin of the range to apply the function to |
| n | - | the number of elements to apply the function to |
| f | - | the function to apply to the projected range [first, first + n)
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| proj | - | projection to apply to the elements |
Return value
An object {first + n, std::move(f)}, where first + n may be evaluated as std::ranges::next(std::move(first), n) depending on iterator category.
Complexity
Exactly n applications of f and proj.
Notes
The overload in namespace::ranges requires Fun to model copy_constructible.
Possible implementation
struct for_each_n_fn { template<std::input_iterator I, class Proj = std::identity, std::indirectly_unary_invocable<std::projected<I, Proj>> Fun> constexpr for_each_n_result<I, Fun> operator()(I first, std::iter_difference_t<I> n, Fun fun, Proj proj = Proj{}) const { for (; n-- > 0; ++first) { std::invoke(fun, std::invoke(proj, *first)); } return {std::move(first), std::move(fun)}; } }; inline constexpr for_each_n_fn for_each_n{};
Example
Run this code
#include <algorithm> #include <array> #include <iostream> #include <ranges> #include <string_view> struct P { int first; char second; friend std::ostream& operator<< (std::ostream& os, const P& p) { return os << '{' << p.first << ",'" << p.second << "'}"; } }; auto print = [](std::string_view name, auto const& v) { std::cout << name << ": "; for (auto n = v.size(); const auto& e: v) { std::cout << e << (--n ? ", " : "\n"); } }; int main() { std::array a{1, 2, 3, 4, 5}; print("a", a); // Negate first three numbers: std::ranges::for_each_n(a.begin(), 3, [](auto& n) { n *= -1; }); print("a", a); std::array s{ P{1,'a'}, P{2, 'b'}, P{3, 'c'}, P{4, 'd'} }; print("s", s); // Negate data members 'pair::first' using projection: std::ranges::for_each_n(s.begin(), 2, [](auto& x) { x *= -1; }, &P::first); print("s", s); // Capitalize data members 'pair::second' using projection: std::ranges::for_each_n(s.begin(), 3, [](auto& c) { c -= 'a'-'A'; }, &P::second); print("s", s); }
Output:
a: 1, 2, 3, 4, 5
a: -1, -2, -3, 4, 5
s: {1,'a'}, {2,'b'}, {3,'c'}, {4,'d'}
s: {-1,'a'}, {-2,'b'}, {3,'c'}, {4,'d'}
s: {-1,'A'}, {-2,'B'}, {3,'C'}, {4,'d'}See also
range-for loop(C++11)
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executes loop over range |
| (C++20) |
applies a function to a range of elements (niebloid) |
| (C++17) |
applies a function object to the first n elements of a sequence (function template) |
| applies a function to a range of elements (function template) |