Core Python Programming (2nd Edition)

8.10. else Statement ... Take Two
In C (as well as in most other languages), you will not find an else statement outside the realm of
conditional statements, yet Python bucks the trend again by offering these in while and for loops. How
do they work? When used with loops, an else clause will be executed only if a loop finishes to
completion, meaning they were not abandoned by break.
One popular example of else usage in a while statement is in finding the largest factor of a number. We
have implemented a function that performs this task, using the else statement with our while loop. The
showMaxFactor() function in Example 8.1 (maxFact.py) utilizes the else statement as part of a while loop.
Example 8.1. while-else Loop Example (maxFact.py)
This program displays the largest factors for numbers between 10 and 20. If the number is
prime, the script will indicate that as well.
1 #!/usr/bin/env python
2
3 def showMaxFactor(num):
4 count = num / 2
5 while count > 1:
6 if num % count == 0:
7 print 'largest factor of %d is %d' % \
8 (num, count)
9 break
10 count -= 1
11 else:
12 print num, "is prime"
13
14 for eachNum in range(10, 21):
15 showMaxFactor(eachNum)
The loop beginning on line 3 in showMaxFactor() counts down from half the amount (starts checking if
two divides the number, which would give the largest factor). The loop decrements each time (line 10)
through until a divisor is found (lines 6-9). If a divisor has not been found by the time the loop
decrements to 1, then the original number must be prime. The else clause on lines 11-12 takes care of
this case. The main part of the program on lines 14-15 fires off the requests to showMaxFactor() with the
numeric argument.
Running our program results in the following output:
largest factor of 10 is 5
11 is prime
largest factor of 12 is 6
13 is prime