fill() and fill_n() functions 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' : 

• 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. 

• 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)