Software Engineering for Students A Programming Approach

■ if-then-else
■ while-do or repeat-until
■ exit
■ method call
7.2 Arguments against goto 91
It is as if the goto is too primitive an instruction – like a machine instruction to load
a register – that can be used in a whole variety of circumstances, but does not clearly
convey its meaning in any of them.
In summary, the goto lacks expressive power and it is therefore difficult to understand
the logic of a program that is written using a lot of gotos. When we look at a
piece of coding, words like while and if give us a strong clue as to what is intended;
gotos do not.
How many pencils?
Suppose we want to read a program in order to understand it by tracing through it as
if we were a computer executing it. Suppose we have available a supply of pencils (or
fingers) to help us. The pencils will be used as markers, and are to be placed on the program
listing to point to places of interest.
If we are following a simple sequence, then we will only need one pencil to keep
track of our position. If we encounter a method call, we need two pencils, one to leave
at the point of the call (in order to know where to return) and another to proceed into
the method.
If we encounter a while statement or a for loop, then we need an integer, a counter,
to keep count of the number of times we have repeated the loop.
To summarize, if the program has been written in a structured way, we need:
■ one pencil to point to the current position
■ one pencil to point to each method call that has been executed but not returned from
■ a counter for every uncompleted loop.
This may seem like a lot of equipment, but consider the alternative of a program that
contains a lot of gotos. As before, we will need to indicate the position of the current
statement. Next, we need a pencil to point at every goto that has been executed. But
now, whereas in the structured program we can remove a pencil whenever we return
from a method, finish a loop or complete an if statement, we can never dispense with
pencils; instead we need ever more. The increased number of pencils reflects the
increased complexity of the goto program.
The real problem becomes evident when we want to refresh our memory about what
happened before we arrived at the current point in the program. In the program without
gotos we simply look back up the program text. In the unstructured program,
when we look backwards we are defeated as soon as we reach a label, because we have
no way of knowing how we reached it.