Software Engineering for Students A Programming Approach

CHAPTER
8 Functional
decomposition
This chapter explains:
■ how to use functional decomposition
■ the principles behind the method.
8.1 ● Introduction
Functional decomposition is a technique for designing either the detailed structure or the
large-scale structure of a program or module. As its name suggests, functional decomposition
is a method that focuses on the functions, or actions, that the software has to carry
out. To use the method we first write down, in English, a single-line statement of what the
software has to do. If the software performs several functions (as most GUI-driven software
does) write down a single line for each function. For example, suppose we want to write a
program to direct a robot to make a cup of instant coffee. We could begin by writing:
make a cup of coffee
Then we express the problem solution in terms of a sequence of simpler actions:
boil water
get a cup
put coffee in cup
add water
add milk
add sugar to taste
Next we take each of these statements and write down what it does in greater detail.
This process of decomposition or refinement is continued until the solution is expressed
in sufficient detail. Usually this is when the solution corresponds to the statements
provided by the programming language to be used.