Software Engineering for Students A Programming Approach

Exercises 109
skill. On the other hand, it is an excellent approach if we want a method that guides
our thinking but allows us plenty of scope for creativity. In a sense, therefore, the
method is not as advanced as some. For example, data structure design takes the programmer
from a description of the structure of the data or files that the program is to
act upon, via a number of fairly precise steps to the program design. By contrast, the
use of functional decomposition encourages (some would say necessitates) the use of
creativity and imagination in devising software structures. Its use also needs careful
judgment in selecting the best structure from amongst any alternatives found.
In summary, functional decomposition is a general-purpose method for software
design, based around structured programming, but in allowing the development of
alternative designs for the same problem it poses several unanswered questions:
■ where do we get the inspiration for alternative designs?
■ how do we choose between designs?
■ how do we know that we have arrived at the best possible design?
We have to look to other sources of ideas to answer these questions. These issues
have led some to say that functional decomposition is not really a serious method.
Summary
Functional decomposition proceeds by starting with a single statement of each
function of the software. Next these are rewritten as a series of simpler steps using
pseudo-code (program design language) as a notation. Pseudo-code consists of
natural language imperative sentences written as sequences, with if statements or
with while statements. The designs are refined (rewritten as more primitive steps)
until the required level of detail is achieved.
The method makes direct use of the power of abstraction provided by structured
programming, while requiring significant creativity and judgment to be
employed. It is applicable to the full range of computer application areas.
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Exercises
8.1 Complete the design of the game program.
8.2 Write pseudo-code for the withdraw cash function in the ATM (Appendix A).
8.3 Use functional decomposition to design the software for each of the systems
described in Appendix A.
8.4 What characteristics should a good software design method have? Does the functional
decomposition exhibit them?