Software Engineering for Students A Programming Approach

360 Chapter 29 ■ Software metrics and quality assurance
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x = y;
if (a == b)
c = d;
else
e = f;
p = q
has a complexity of 2, because there are two independent paths through the program.
Similarly a while and a repeat each count one towards the complexity count.
Compound conditions like:
if a > b and c > d then
count two because this if statement could be rewritten as two, nested if statements.
Note that a program that consists only of a sequence of statements, has a cyclomatic
complexity of 1, however long it is. Thus the smallest value of this metric is 1.
There are two ways of using McCabe’s measure. First, if we had two algorithms that
solve the same problem, we could use this measure to select the simpler. Second, McCabe
suggests that if the cyclomatic complexity of a component is greater than 10, then it is
too complex. In such a case, it should either be rewritten or else broken down into several
smaller components.
Cyclomatic complexity is a useful attempt to quantify complexity, and it is claimed
that it has been successfully applied. It is, however, open to several criticisms as follows.
First, why is the value of 10 adopted as the limit? This figure for the maximum
allowed complexity is somewhat arbitrary and unscientific.
Second, the measure makes no allowance for the sheer length of a module, so that a
one-page module (with no decisions) is rated as equally complex as a thousand-page
module (with no decisions).
Third, the measure depends only on control flow, ignoring, for example, references
to data. One program might only act upon a few items of data, while another might
involve operations on a variety of complex objects. (Indirect references to data, say via
pointers, are an extreme case.)
Finally, there is no evidence to fully correlate McCabe’s measure with the complexity
of a module as perceived by human beings.
So McCabe’s measure is a crude attempt to quantify the complexity of a software
component. But it suffers from obvious flaws and there are various suggestions for
devising an improved measure. However, McCabe’s complexity measure has become
famous and influential as a starting point for work on metrics.
SELF-TEST QUESTION
29.1 Suggest a formula for calculating the complexity of a piece of program.
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