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

Fill() and Fill_n() are two functions in the C++ Standard Template Library (STL) that are used to fill a range of elements in a container with a given value. However, they differ somewhat in terms of functionality and use. In this article, we will discuss difference between Fill() and Fill_n() method in C++. But before discussing their differences, we must know about the Fill() and Fill_n() method.

What is the std::fill() method?

The specified value is filled in the range [first, last) by std::fill().
It requires two iterators, first and last, which indicate the range that needs to be filled and a value that will be applied to each element.

Syntax:

It has the following syntax:

template<class ForwardIt, class T>
void fill(ForwardIt first, ForwardIt last, const T& value);

Example:

Let us take an example to illustrate the std::fill() method in C++.

#include <algorithm>
#include <vector>
#include <iostream>
int main() {
std::vector<int> numbers = {7, 9, 8, 1, 1};
std::fill(numbers.begin(), numbers.end(), 5);
for (const auto& num : numbers) {
std::cout << num << " ";
}
return 0;
}

Output:

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

What is the std::fill_n() method?

The std::fill_n() method inserts the given value into the first count elements, beginning with the iterator.
Filling the elements requires an iterator that points to the beginning position, the number of elements to fill (count), and a value.

Syntax:

It has the following syntax:

template<class OutputIt, class Size, class T>
OutputIt fill_n(OutputIt first, Size count, const T& value);

Example:

Let us take an example to illustrate the std::fill_n() method in C++.

#include <algorithm>
#include <vector>
#include <iostream>
int main() {
std::vector<int> numbers = {7, 9, 8, 1, 1};
std::fill_n(numbers.begin(), 3, 5);
for (const auto& num : numbers) {
std::cout << num << " ";
}
return 0;
}

Output:

Main differences between fill() and fill_n() method:

There are several differences between the fill() and fill_n() method. Some main differences between these methods are as follows:

S.No	fill()	fill_n()
1.	It sets the given value to every array element.	It is employed to give a new value to a predetermined number of elements in a range that start at a specific 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.	There is no value returned by it.	It returns an iterator pointing to the element that comes after the final element to be filled (in C++ 11). (In C++ 98) yields null.
4.	O(N) is the time complexity of it.	O(N) is the time complexity of it.