# std::for_each_n

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
| (1) |
template< class InputIt, class Size, class UnaryFunction > InputIt for_each_n( InputIt first, Size n, UnaryFunction f ); |  | (since C++17) (until C++20)
template< class InputIt, class Size, class UnaryFunction > constexpr InputIt for_each_n( InputIt first, Size n, UnaryFunction f ); |  | (since C++20)
template< class ExecutionPolicy, class ForwardIt, class Size, class UnaryFunction2 > ForwardIt for_each_n( ExecutionPolicy&& policy, ForwardIt first, Size n, UnaryFunction2 f ); | (2) | (since C++17)

1) Applies the given function object `f` to the result of dereferencing every iterator in the range `[first, first + n)`, in order.
2) Applies the given function object `f` to the result of dereferencing every iterator in the range `[first, first + n)` (not necessarily in order). The algorithm is executed according to `policy`. This overload does not participate in overload resolution unless std::is_execution_policy_v < std::decay_t <ExecutionPolicy>> is true.

For both overloads, 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.

### 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. See execution policy for details.
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.
- `UnaryFunction` must meet the requirements of MoveConstructible. Does not have to be CopyConstructible
- `UnaryFunction2` must meet the requirements of CopyConstructible.

### Return value

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

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

### Example

Run this code

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

    int main()
    {
        std::vector<int> ns{1, 2, 3, 4, 5};
        for (auto n: ns) std::cout << n << ", ";
        std::cout << '\n';
        std::for_each_n(ns.begin(), 3, [](auto& n){ n *= 2; });
        for (auto n: ns) std::cout << n << ", ";
        std::cout << '\n';
    }

Output:

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

### See also

transform | applies a function to a range of elements (function template)
range-for loop | executes loop over range (since C++11)
for_each | applies a function to a range of elements (function template)

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