std::fill_n

Defined in header <algorithm>

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)

Assigns the given value to the first count elements in the range beginning at first if count > 0. Does nothing otherwise.

Parameters

first	-	the beginning of the range of elements to modify
count	-	number of elements to modify
value	-	the value to be assigned
Type requirements
- `OutputIt` must meet the requirements of `OutputIterator`.

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.

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:

#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 -1 5 6 7 8 9`

fill	assigns a range of elements a certain value (function template)
std::experimental::parallel::fill_n (parallelism TS)	parallelized version of `std::fill_n` (function template)