Subroutine and Functions:

 In a program, it is often necessary to repeat a statement or group of statements at several points to accomplish a particular task. Repeating the same statement in a program each time makes a program lengthy and reduces readability. These problems could be sorted out if the necessary statements could be written once and then referred to each time they are needed. This is the purpose of a subprogram. Basically there are two different types of subprograms, called functions and subroutines.  Making subprograms allows tackling small pieces of a problem individually. 

Once each piece is working correctly then the pieces are integrated together to create the complete solution of the problem. To implement functions and subroutines, we require writing the main program that references all of the subprograms in the desired order and also writing the subprograms. This can  be done in any order that is convenient.