fill in C++ STL

std::fill is a C++ STL algorithm defined in the <algorithm> header that assigns a specified value to every element in a given range [first, last). It works with arrays and STL containers that provide at least forward iterators (such as vector, list, and deque).

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

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

    // Fill elements from index 2 to index 6 with value 4
    fill(v.begin() + 2, v.end() - 1, 4);

    for (int x : v)
        cout << x << " ";

    return 0;
}

0 0 4 4 4 4 4 0 

Explanation: This code fills the elements from index 2 to 6 of the vector with the value 4 using std::fill.

Syntax

std::fill(first, last, value);

Parameters

• first: Iterator pointing to the first element of the range.
• last: Iterator pointing to the position just after the last element of the range.
• value: The value to be assigned to each element in the range.

Note: The range is half-open — first is included, but last is excluded.

Example 1: Using std::fill with an Array 

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

int main() {
    int arr[10];

    // Fill entire array with value 4
    fill(arr, arr + 10, 4);

    for (int i = 0; i < 10; i++)
        cout << arr[i] << " ";

    return 0;
}

4 4 4 4 4 4 4 4 4 4 

Explanation: This code assigns the value 4 to all elements of the array by applying std::fill on the entire array range.

Example 2: Using std::fill with a List

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

int main() {
    list<int> ml = {10, 20, 30};

    // Fill all elements of the list with value 4
    fill(ml.begin(), ml.end(), 4);

    for (int x : ml)
        cout << x << " ";

    return 0;
}

4 4 4 

Explanation: This code updates every element of the list to 4 by calling std::fill from begin() to end()