The while statement

The while statement
	Chapter 6. Control Flow

The while statement allows you to repeatedly execute a block of statements as long as a condition is true. A while statement is an example of what is called a looping statement. A while statement can have an optional else clause.

Using the while statement

Example 6.2. Using the while statement

				
#!/usr/bin/python
# Filename: while.py

number = 23
running = True

while running:
	guess = int(raw_input('Enter an integer : '))

	if guess == number:
		print 'Congratulations, you guessed it.'
		running = False # this causes the while loop to stop
	elif guess < number:
		print 'No, it is a little higher than that.'
	else:
		print 'No, it is a little lower than that.'
else:
	print 'The while loop is over.'
	# Do anything else you want to do here

print 'Done'

Output

				
$ python while.py
Enter an integer : 50
No, it is a little lower than that.
Enter an integer : 22
No, it is a little higher than that.
Enter an integer : 23
Congratulations, you guessed it.
The while loop is over.
Done

How It Works

In this program, we are still playing the guessing game, but the advantage is that the user is allowed to keep guessing until he guesses correctly - there is no need to repeatedly execute the program for each guess as we have done previously. This aptly demonstrates the use of the while statement.

We move the raw_input and if statements to inside the while loop and set the variable running to True before the while loop. First, we check if the variable running is True and then proceed to execute the corresponding while-block. After this block is executed, the condition is again checked which in this case is the running variable. If it is true, we execute the while-block again, else we continue to execute the optional else-block and then continue to the next statement.

The else block is executed when the while loop condition becomes False - this may even be the first time that the condition is checked. If there is an else clause for a while loop, it is always executed unless you have a while loop which loops forever without ever breaking out!

The True and False are called Boolean types and you can consider them to be equivalent to the value 1 and 0 respecitvely. It's important to use these where the condition or checking is important and not the actual value such as 1.

The else-block is actually redundant since you can put those statements in the same block (as the while statement) after the while statement to get the same effect.

Note for C/C++ Programmers

Remember that you can have an else clause for the while loop.