# std::for_each_n

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class InputIt, class Size, class UnaryFunc > InputIt for_each_n( InputIt first, Size n, UnaryFunc f ); | (1) | (since C++17) (constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt, class Size, class UnaryFunc > ForwardIt for_each_n( ExecutionPolicy&& policy, ForwardIt first, Size n, UnaryFunc f ); | (2) | (since C++17)

Applies the given function object f to the result of dereferencing every iterator in the range `[`first`,` first + n`)`. If f returns a result, the result is ignored.

1) f is applied in order starting from first.
If `UnaryFunc` is not MoveConstructible, the behavior is undefined.
2) f might not be applied in order. The algorithm is executed according to policy.
This overload participates in overload resolution only if all following conditions are satisfied:

std::is_execution_policy_v < std::decay_t <ExecutionPolicy>> is true. | (until C++20)
std::is_execution_policy_v < std::remove_cvref_t <ExecutionPolicy>> is true. | (since C++20)

If `UnaryFunc` is not CopyConstructible, the behavior is undefined.

If n >= 0 is not true, the behavior is undefined.

If the iterator type (`InputIt`/`ForwardIt`) is mutable, f may modify the elements of the range through the dereferenced iterator.

Unlike the rest of the parallel algorithms, `for_each_n` is not allowed to make copies of the elements in the sequence even if they are TriviallyCopyable.

### Parameters

first | - | the beginning of the range to apply the function to
n | - | the number of elements to apply the function to
policy | - | the execution policy to use
f | - | function object, to be applied to the result of dereferencing every iterator in the range `[`first`,` first + n`)` The signature of the function should be equivalent to the following: void fun(const Type &a); The signature does not need to have const &. The type Type must be such that an object of type InputIt can be dereferenced and then implicitly converted to Type. ​
Type requirements
- `InputIt` must meet the requirements of LegacyInputIterator.
- `ForwardIt` must meet the requirements of LegacyForwardIterator.
- `Size` must be convertible to an integral type.

### Return value

An iterator equal to first + n, or more formally, to std::advance (first, n).

### Complexity

Exactly n applications of f.

### Exceptions

The overload with a template parameter named `ExecutionPolicy` reports errors as follows:

- If execution of a function invoked as part of the algorithm throws an exception and `ExecutionPolicy` is one of the standard policies, std::terminate is called. For any other `ExecutionPolicy`, the behavior is implementation-defined.
- If the algorithm fails to allocate memory, std::bad_alloc is thrown.

### Possible implementation

See also the implementation in libstdc++, libc++ and MSVC stdlib.

    template<class InputIt, class Size, class UnaryFunc>
    InputIt for_each_n(InputIt first, Size n, UnaryFunc f)
    {
        for (Size i = 0; i < n; ++first, (void) ++i)
            f(*first);

        return first;
    }

### Example

Run this code

    #include <algorithm>
    #include <iostream>
    #include <vector>

    void println(auto const& v)
    {
        for (auto count{v.size()}; const auto& e : v)
            std::cout << e << (--count ? ", " : "\n");
    }

    int main()
    {
        std::vector<int> vi{1, 2, 3, 4, 5};
        println(vi);

        std::for_each_n(vi.begin(), 3, [](auto& n) { n *= 2; });
        println(vi);
    }

Output:

    1, 2, 3, 4, 5
    2, 4, 6, 4, 5

### See also

transform | applies a function to a range of elements, storing results in a destination range (function template)
range-`for` loop (C++11) | executes loop over range
for_each | applies a function to a range of elements (function template)
ranges::for_each_n (C++20) | applies a function object to the first N elements of a sequence (algorithm function object)

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