Page 307 - Computer Science V2.0 Class 11

11.4 while Statement

The while statement is used to execute a sequence of statements over and over again as long as the condition
specified in the while statement is True. The sequence of statements that is executed repeatedly is called body of the
while statement. The while statement has the following syntax:

while test condition:
Body of while loop
[else:
Statements]
In the above syntax description:
while is a keyword

test condition is the expression that will evaluate to either True or False
body of while loop constitutes the statement(s) that will be executed if the test condition is True. These are
determined through indentation. The first un-indented line marks the end of the loop.
else (optional) is a keyword and the statements in else block (optional) will be executed after all the possible
iterations of the while loop are executed.

We first discuss the while statement, ignoring the optional else clause. Fig 11.3 shows a flowchart that depicts
the execution of such a while statement, Note that the Boolean expression (also called a conditional expression,
test expression, or just a condition) in the while statement is evaluated before the body of the while statement
is executed. If the Boolean expression yields False, the loop's body is skipped, and the execution of the while
statement ends. However, if the Boolean expression yields True, the body of the while statement is executed. On
executing the loop's body, the Boolean expression is evaluated again. If the Boolean expression yields True, the
loop's body is executed again. This process of evaluating the Boolean expression and executing the body of the while
statement is repeated until the Boolean expression becomes False.

Enter while loop

Test False
Expression

True

Body of while

Exit loop
Fig 11.3: while statement

11.4.1 Summing the Digits of a Number

Now we are ready to study an application of the while statement. Suppose we wish to find the sum of digits of a
number.
In Lines 22–23, we write a function sumofDigits(num) that accepts as input parameter the number num.

On execution of Line 13, we ignore the sign of the number by taking its absolute value.

Looping in Python 293