# std::fill_n

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
| (1) |
template< class OutputIt, class Size, class T > void fill_n( OutputIt first, Size count, const T& value ); |  | (until C++11)
template< class OutputIt, class Size, class T > OutputIt fill_n( OutputIt first, Size count, const T& value ); |  | (since C++11) (until C++20)
template< class OutputIt, class Size, class T > constexpr OutputIt fill_n( OutputIt first, Size count, const T& value ); |  | (since C++20)
template< class ExecutionPolicy, class ForwardIt, class Size, class T > ForwardIt fill_n( ExecutionPolicy&& policy, ForwardIt first, Size count, const T& value ); | (2) | (since C++17)

1) Assigns the given `value` to the first `count` elements in the range beginning at `first` if `count > 0`. Does nothing otherwise.
2) Same as (1), but executed according to `policy`. This overload only participates in overload resolution if std::is_execution_policy_v < std::decay_t <ExecutionPolicy>> is true

### Parameters

first | - | the beginning of the range of elements to modify
count | - | number of elements to modify
value | - | the value to be assigned
policy | - | the execution policy to use. See execution policy for details.
Type requirements
- `OutputIt` must meet the requirements of LegacyOutputIterator.
- `ForwardIt` must meet the requirements of LegacyForwardIterator.

### Return value

(none) | (until C++11)
Iterator one past the last element assigned if `count > 0`, `first` otherwise. | (since C++11)

### Complexity

Exactly `count` assignments, for `count > 0`.

### 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 OutputIt, class Size, class T>
    OutputIt fill_n(OutputIt first, Size count, const T& value)
    {
        for (Size i = 0; i < count; i++) {
            *first++ = value;
        }
        return first;
    }

### Example

The following code uses `fill_n()` to assign -1 to the first half of a vector of integers:

Run this code

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

    int main()
    {
        std::vector<int> v1{0, 1, 2, 3, 4, 5, 6, 7, 8, 9};

        std::fill_n(v1.begin(), 5, -1);

        std::copy(begin(v1), end(v1), std::ostream_iterator<int>(std::cout, " "));
        std::cout << "\n";
    }

Output:

    -1 -1 -1 -1 -1 5 6 7 8 9

### See also

fill | copy-assigns the given value to every element in a range (function template)

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