std::fill_n() function with example in C++ STL

C++ STL | std::fill_n() function: Here, we are going to learn about the fill_n() function of algorithm header in C++ STL with example.
Submitted by IncludeHelp, on May 21, 2019

C++ STL std::fill_n() function

fill_n() function is a library function of algorithm header, it is used to assign a value to the n elements of a container, it accepts an iterator pointing to the starting position in the container, n (number of elements) and a value to be assigned to the n elements, and assigns the value.

Note: To use fill_n() function – include <algorithm> header or you can simple use <bits/stdc++.h> header file.

Syntax of std::fill_n() function

    std::fill_n(iterator start, n, value);

Parameter(s):

iterator start – an iterator pointing to the position from where we have to assign the value to the next n elements.
n – number of elements to be assigned with the given value.
value – a value of the same type to be assigned to the n elements.

Return value: void – it returns noting.

Example:

    Input:
    vector<int> v(10);
    
    //filling 10 elements with -1
    fill(v.begin(), 10, -1);
    
    Output:
    -1 -1 -1 -1 -1 -1 -1 -1 -1 -1

C++ STL program to demonstrate use of std::fill_n() function

In this program, we are going to fill the n elements of a vector.

//C++ STL program to demonstrate use of
//std::fill_n() function
#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;

int main()
{
    //vector
    vector<int> v(10);

    //filling all elements with -1
    fill_n(v.begin(), 10, -1);

    //printing vector elements
    cout << "v: ";
    for (int x : v)
        cout << x << " ";
    cout << endl;

    //filling initial 3 elements with 100
    fill_n(v.begin(), 3, 100);

    //printing vector elements
    cout << "v: ";
    for (int x : v)
        cout << x << " ";
    cout << endl;

    //filling rest of the elements with 200
    fill_n(v.begin() + 3, 7, 200);

    //printing vector elements
    cout << "v: ";
    for (int x : v)
        cout << x << " ";
    cout << endl;

    return 0;
}

Output

v: -1 -1 -1 -1 -1 -1 -1 -1 -1 -1
v: 100 100 100 -1 -1 -1 -1 -1 -1 -1
v: 100 100 100 200 200 200 200 200 200 200

Reference: C++ std::fill_n()