Chapter 4: All About Functions

Dealing with Complexity

In Chapter 3, the programs we wrote only consisted of one section of code. However, if we wrote real programs like that, it would be impossible to maintain them. It would be really difficult to get multiple people working on the project, as any change in one part might adversely affect another part that another developer is working on.

To assist programmers in working together in groups, it is necessary to break programs apart into separate pieces, which communicate with each other through well-defined interfaces. This way, each piece can be developed and tested independently of the others, making it easier for multiple programmers to work on the project.

Programmers use functions to break their programs into pieces which can be independently developed and tested. Functions are units of code that do a defined piece of work on specified types of data. For example, in a word processor program, I may have a function called handle_typed_character which is activated whenever a user types in a key. The data the function uses would probably be the keypress itself and the document the user currently has open. The function would then modify the document according to the keypress it was told about.

The data items a function is given to process are called its parameters. In the word processing example, the key which was pressed and the document would be considered parameters to the handle_typed_characters function. The parameter list and the processing expectations of a function (what it is expected to do with the parameters) are called the function's interface. Much care goes into designing function interfaces, because if they are called from many places within a project, it is difficult to change them if necessary.

A typical program is composed of hundreds or thousands of functions, each with a small, well-defined task to perform. However, ultimately there are things that you cannot write functions for which must be provided by the system. Those are called primitive functions (or just primitives) - they are the basics which everything else is built off of. For example, imagine a program that draws a graphical user interface. There has to be a function to create the menus. That function probably calls other functions to write text, to write icons, to paint the background, calculate where the mouse pointer is, etc. However, ultimately, they will reach a set of primitives provided by the operating system to do basic line or point drawing. Programming can either be viewed as breaking a large program down into smaller pieces until you get to the primitive functions, or incrementally building functions on top of primitives until you get the large picture in focus. In assembly language, the primitives are usually the same thing as the system calls, even though system calls aren't true functions as we will talk about in this chapter.