Programming languages and software engineering - The Computing Universe: A Journey through a Revolution - page 62

Fig. 4.4. Flowcharts for the “for” loop (a)
and “if-then-else” (b). They illustrate the
decision points in the program where
a branch can be made depending on a
check of some condition. The “for” loop
performs a set number of iterations
through the same piece of code that
use different data. The “if-then-else”
branch executes a different piece of
code depending on the result of a test
condition.

(a) (b)

no

More
iterations?

yes no

Condition
true?

yes

Statements#1

Statements#2

Statements

it, however, because the pseudocode version of the program omits details not
essential for understanding by people but necessary for a machine to run the
program. Thus we can write a do loop using the for keyword

for <var> in <sequence>
<statements>

The variable <var> after the keyword for is called the loop index , and the state-
ments in the body of the for loop - <statements> - are executed according to
the number of times specified in the <sequence> portion of the loop heading.
After completing the loop the required number of times, the program goes to
the program statement after the last statement in the body of the loop.
Similarly, we can write an if statement as:

if <condition>:
<statements#1>
else:
<statements#2>

If the Boolean condition <condition> is true, the program executes the first set
of statements - <statements#1> - and then jumps to the statement after the
second block of statements - <statements#2>. If the condition is false, the pro-
gram jumps over the first block of statements and executes the second block
of code <statements#2> under the else keyword, before carrying on with the
next statement of the program. It is sometimes helpful to visualize these con-
trol structures as diagrams called lowcharts , first introduced into computing
by Herman Goldstine and John von Neumann in 1947. Flowcharts use boxes
to represent the different parts of the program with arrows between the boxes
showing the sequence of events. Flowcharts representing the for loop and the
if-then-else flowcharts are shown in Fig. 4.4 .
Modern programming languages generally do not encourage use of the go
to statement. Undisciplined use of go to statements often led to very complex
“spaghetti” code that was difficult to understand and debug. In 1968, Edsger
Dijkstra ( B.4.2 ) wrote a famous article titled “Go To Statement Considered
Harmful”: