C++ fill_n() function explanation with example

C++ fill_n() function explanation with example:

fill_n is an important method in C++. This method is used to fill upto n positions from a starting position in a container. It accepts one integer or the starting position, the number of positions to fill i.e. n and the value to fill in.

In this post, we will learn how to use fill_n, its syntax and example of fill_n.

How to use fill_n() function:

To use fill_n, you need to import or include the header. You can also use <bits/stdc++.h> header.

syntax of fill_n:

fill_n is defined as like below:

std::fill_n(iterator s, n, v)

Parameters:

It takes three parameter: s, n and v.

  • s is an iterator that points to the start position, i.e. it defines the position from where we need to start the filling.
  • n is the number of positions to fill.
  • v is the value we are filling.

Return value of fill_n:

fill_n method returns void, i.e. it doesn’t return anything.

Example of fill_n:

Let’s try to use fill_n with an example. We will use vector in this example.

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

using namespace std;

int main()
{
    vector<char> givenVector(5);
    
    fill_n(givenVector.begin(), 5, '*');
    
    for(char c: givenVector)
        cout<<c<<" ";

    return 0;
}

Here,

  • givenVector is a vector that can hold characters and its size is 5.
  • We are using fill_n on this vector.

    • It starts from the beginning of the vector, i.e. the iterator is starting from the start of the vector
    • It fills 5 values of the vector
    • The fill character is *.
  • The for loop iterates through the characters of the vector one by one and prints them.

Output:

If you run this program, it will print the below output:

* * * * *

As you can see, all positions are filled with * in the vector.

Example of fill_n from a specific index:

We can also fill the values of the vector from a specific position. Let’s take a look at the below example:

#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;

int main()
{
  vector<int> v(8);

  fill_n(v.begin() + 3, 5, -3);

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

  return 0;
}

Here,

  • We are using fill_n to fill integers from index 3 and it is filling 5 positions for the vector v.
  • v is a vector that can hold only integer values.
  • The for loop is printing the values of the vector.

If you run this program, it will print the below output:

0 0 0 -3 -3 -3 -3 -3

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