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Showing content from https://www.geeksforgeeks.org/cpp/fill-and-fill_n-functions-in-c-stl/ below:

fill() and fill_n() functions in C++ STL

Last Updated : 23 Jul, 2025

A vector, once declared, has all its values initialized to zero. Following is an example code to demonstrate the same.

CPP


 // C++ program for displaying the default initialization
// of the vector vect[]
#include<bits/stdc++.h>
using namespace std;

int main() 
{
    // Creating a vector of size 8
    vector<int> vect(8);  

    // Printing default values
    for (int i=0; i<vect.size(); i++)
       cout << ' ' << vect[i];
}

Output :

0 0 0 0 0 0 0 0

What if we wish to initialize the vector to a specific value, say 1 ? For this, we can pass the value along with the size of the vector.

CPP


 // C++ program for displaying specified initialization
// of the vector vect[]
#include<bits/stdc++.h>
using namespace std;

int main () 
{
    // Creates a vector of size 8 with all initial
    // values as 1.
    vector<int> vect(8, 1);  

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

Output :

1 1 1 1 1 1 1 1

What if we wish to initialize the first 4 values to say 100 and rest 6 values as 200 ? One way to do this is to manually provide a value to each position in the vector. The other methods as provided in STL, the Standard Template Library, are fill and fill_n.

fill() The 'fill' function assigns the value 'val' to all the elements in the range [begin, end), where 'begin' is the initial position and 'end' is the last position. NOTE : Notice carefully that 'begin' is included in the range but 'end' is NOT included. Below is an example to demonstrate 'fill' :

CPP


 // C++ program to demonstrate working of fill()
#include <bits/stdc++.h>
using namespace std;

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

  // calling fill to initialize values in the
  // range to 4
  fill(vect.begin() + 2, vect.end() - 1, 4);

  for (int i=0; i<vect.size(); i++)
    cout << vect[i] << " ";

  return 0;
}

Output :

0 0 4 4 4 4 4 0

fill_n() In fill_n(), we specify beginning position, number of elements to be filled and values to be filled. The following code demonstrates the use of fill_n.

CPP


 // C++ program to demonstrate working of fil_n()
#include <bits/stdc++.h>
using namespace std;

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

    // calling fill to initialize first four values
    // to 7
    fill_n(vect.begin(), 4, 7);

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

    // calling fill to initialize 3 elements from 
    // "begin()+3" with value 4
    fill_n(vect.begin() + 3, 3, 4);

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

    return 0;
}

Output :

 7 7 7 7 0 0 0 0
 7 7 7 4 4 4 0 0

Let us see the difference table in a tabular form -:

fill() fill_n() 1. It sets given value to all elements of array. It is used to assign a new value to a specified number of elements in a range beginning with a particular element. 2.

Its syntax is -:

void fill(ForwardIterator first, ForwardIterator last, const value_type &val);

Its syntax is -:

In C++ 98:

void fill_n (OutputIterator first, Size n, const T& val);

From C++11 onwards:

OutputIterator fill_n (OutputIterator first, Size n, const T& val);

3. It has no return value.

(In C++ 11)It returns an iterator pointing to the element that follows the last element to be filled.

(In C++ 98) returns none.

4. Its time complexity is O(N) Its time complexity is O(N)

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