A fundamental approach to solving large, complex problems is to break the problem down into smaller problems. Then, solutions can be found for these smaller problems and organized in such a manner as to address the original problem. In a similar way, problems that require long, complex FORTRAN code can be broken down into program units. In addition to the main program unit, FORTRAN programs can contain subprograms. Subprograms allow the programmer to organize programs in a logical, hierarchical fashion. The FORTRAN language provides for two types of subprograms: functions and subroutines. A function returns a single value; a subroutine may return one or more values. There are several advantages to using subprograms:

• they help keep the code simple, and, thus, more readable;
• they allow the programmer to use the same code as many times as needed throughout the program;
• they allow the programmer to define needed functions; and,
• they can be used in other programs.

• read the data from a file;
• perform calculations based on the data read;
• sort the data; and,
• save the data in a new file.