std::fill() and fill_n() in C++ STL

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

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.

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’ :

  // 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.

  // 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 << ' ';        // 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 << ' ';        return 0; }

  Output :

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

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