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How to use the fill_n() function in C++

Vinisha Maheshwari

In this shot, we will discuss how to use the fill_n() function. The fill_n() function is available in the <bits/stdc++.h> header file in C++ and is used to fill values up to the first n positions from a starting position.


void fill_n (Begin, End, Value);


The fill_n() function takes the following parameters:

  • Begin: Iterator pointing to the beginning of the position from which the values need to be filled.
  • End: Number of positions up until which the value needs to be filled.
  • Value: The value to be filled.

Return value

fill_n() does not return a value.


Let’s understand with the help of an example:

  • We have a vector of size 7.

  • We use the fill_n(vector.begin,4,3) function to fill the value (3) up to the 4th position.

  • The vector is 3 3 3 3 0 0 0 after we use the fill_n() function.


Let’s look at the code snippet below.

#include <bits/stdc++.h>
using namespace std;

int main()
vector<int> v(7);
fill_n(v.begin(), 4, 5);

for (int i = 0; i < v.size(); i++)
cout << ' ' << v[i];
cout << '\n';

return 0;    
Using the fill_n() function in C++


  • In line 1, we import the required header file.

  • In line 4, we make a main function.

  • In line 6, we declare a vector of int data type of size 7.

  • In line 7, we use the fill_n() function to fill the value 5 from the beginning of the vector to the 4th position.

  • In lines 9 and 10, we use the for loop to access the elements of the vector and display them as a result.

In this way, we can use the fill_n() function in C++.



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