Software Engineering for Students A Programming Approach

90 Chapter 7 ■ Structured programming
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listing of a program that is written in a structured way and asked a series of questions
that are designed to assess their comprehension of it. The accuracy of the replies and
the time taken to reply are both measured. These are measures of the ease with which
the program could be debugged or changed. A second group of subjects are given
copies of the same program rewritten in an unstructured way. The accuracy and
response times of the two groups are compared. The results of these experiments generally
indicate that structured programs are superior to unstructured ones.
The results of empirical studies are reviewed in the literature given at the end of
the chapter. In a review published in 1984, long after the dust had settled on the
structured programming debate, Vessey and Weber concluded that “the evidence
supporting [structured programming] is weak”. This conclusion largely stems from
the difficulty of carrying out experiments that give trustworthy results.
Clarity and expressive power
Compare the following two equivalent program fragments:
------ -------
------ ------label:
while a > 0
------ -------
------ ------if
a > 0 goto label endwhile
------ -------
------ -------
As we read down the first program fragment, we are not immediately sure what the
roles of the label and goto are. It would take us some time to read and study the program
in order to discover that they are being used to create the repetition of a piece of
code. This is made immediately obvious by the while statement in the second program.
Worse, there is a remaining doubt in the first program that there may be another
goto aimed at this same label from some other point in the program.
The facilities of a programming language should allow people to describe what they
want to do in a meaningful way. If we examine a typical program written using gotos
we see that the gotos are used for a variety of purposes, for example:
■ to avoid a piece of code (which is to be executed in different circumstances)
■ to perform repetition
■ to exit from the middle of a loop
■ to invoke a shared piece of code.
When we see a goto, there are few clues that allow us to decide the purpose for
which the goto is being used. The alternative is, of course, a unique language construct
for use in each of these different circumstances. These are, respectively:
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