See also Programs are Algorithms and Program Structures

Modular Programming

• Decomposing large or complex programs into modules.
• Each module is a part of the larger program.
• A program becomes a set of modules.
• Each module can be easily understood, designed, implemented, and maintained

by different programmers, or teams of programmers.

• Modules are sometimes called subroutines, procedures, or even functions.
• A module can be seen as a part of a program which performs a particular logical task.

1. Modular programs are hierarchical.  That is to say, there are higher and the lower level modules. (see diagram below)
2. The top-level Control Module makes all major decisions regarding flow of control through the program. (see Pseudocode below)
3. Modules have only one entry point and one exit point (excepting legal violations.  See Program Structures).
4. A module can only be invoked (called) from a high level modules.
5. On exiting a module control is returned to the statement immediately following the invocation.

Pseudocode
       Start  Program
              Start Control Module
                     Do Initialise
                     Do Program Body
                     Do Terminate
              End control module.

               Procedure Initialise
                     statements .....
                 End Procedure
               Procedure Program Body
                     statements .....
                 End Procedure
               Procedure Terminate
                     statements .....
                 End Procedure
               etc.
               etc.
      End Program