# std::ranges::for_each_n, std::ranges::for_each_n_result

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< cpp‎ | algorithm‎ | ranges

Defined in header `<algorithm>` |  |
Call signature |  |
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> for_each_n( I first, std::iter_difference_t<I> n, Fun f, Proj proj = {}); | (1) | (since C++20)
Helper types |  |
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 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.

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`)`
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.

### 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 'P::first' using projection:
        std::ranges::for_each_n(s.begin(), 2, [](auto& x) { x *= -1; }, &P::first);
        print("s", s);
        // Capitalize data members 'P::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) | executes loop over range
ranges::for_each (C++20) | applies a function to a range of elements (niebloid)
for_each_n (C++17) | applies a function object to the first n elements of a sequence (function template)
for_each | applies a function to a range of elements (function template)

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