Software Engineering for Students A Programming Approach

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14.7 Repetition 183
There is something of a controversy here. Some people argue that when a case statement
is executed, the programmer should be completely aware of all the possibilities that
can occur. So the default statement is redundant and just an invitation to be lazy and
sloppy. Where necessary, the argument goes, a case statement should be preceded by
if statements that ensure that only valid values are supplied to the case statement.
if-not
It would be reasonable to think that there would no longer be any controversy over language
structures for selection. The if-else is apparently well established. However,
the lack of symmetry in the if statement is open to criticism. While it is clear that the
then part is carried out if the condition is true, the else part is rather tagged on at the
end to cater for all other situations. Experimental evidence suggests that significantly
fewer bugs arise if the programmer is required to restate the condition (in its negative
form) prior to the else as shown below:
if condition
statement1
not condition else
statement2
endif
14.7 ● Repetition
Control structures for repetition traditionally fall into two classes. There are loop structures
where the number of iterations is fixed, and those where the number of iterations
is controlled by the evaluation of some condition. Fixed length iteration is often implemented
using a form similar to that shown below:
for control_variable =
initial_expression to final_expression step step_expression
do
statement(s)
endfor
The usefulness and reliability of the for statement can be affected by a number of
issues as now discussed
Should the type of the loop control variable be limited to integers? Perhaps any ordinal
type should be allowed. However, reals (floats) should not be allowed. For example,
consider how many iterations are specified by the following:
for x = 0.0 to 1.0 step 0.33 do
Here it is not at all obvious exactly how many repetitions will be performed, and things
are made worse by the fact that computers represent real values only approximately.
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