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9.4 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 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 9.3: while statement
9.4.1 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. For this purpose, we write a program that accepts as input the number num, whose sum of digits we wish
to find (see Program 9.5). As we are only interested in finding the sum of the digits of num, we ignore the sign of the
number by taking its absolute value (line 14). Next, we initialise the sum of the digits (digitSum) as zero. If the
number whose sum of digits we want to find is zero, the condition num > 0 fails at the beginning of the execution
of the while statement. In this case, the execution of the while statement finishes without executing the body of
the while statement even once. However, if the condition num > 0 yields True at the beginning of the while
statement, we find the digits one by one beginning at the unit's place.
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