Modular programming and Modules Modular programming is breaking down the design of a program into individual components (modules) that can be programmed and tested independently. It is a requirement for effective development and maintenance of large programs and projects. With modular programming, procedures of a common functionality are grouped together into separate modules. A program therefore no longer consists of only one single part. It is now divided into several smaller parts which interact and which form the whole program. Definition of Functions Modules in C++ are called functions. A function is a subprogram that can act on data and return a value. Every C++ program has at least one function, main(). When your program starts, main() is called automatically. main() might call other functions, some of which might call still others. Each function has its own name, and when that name is encountered, the execution of the program branches to the body of that function. When the function returns, execution resumes on the next line of the calling function. When a program calls a function, execution switches to the function and then resumes at the line after the function call. Welldesigned functions perform a specific and easily understood task. Complicated tasks should be broken down into multiple functions, and then each can be called in turn. Functions come in two varieties: user-defined and built-in. Built-in functions are part of your compiler package--they are supplied by the manufacturer for your use. In this chapter we will discuss about user-defined functions.