Control Flow (while)

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.

Example:

def guessing_game(n):
  running = True
  while running:
      guess = int(input('Enter an integer : '))
  
      if guess == number:
          print('Congratulations, you guessed it.')
          # this causes the while loop to stop
          running = False
      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

def main():
  number = 23
  guessing_game(number)
  print('Done')
  
main()

Output:

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 run the program for each guess, as we have done in the previous section. This aptly demonstrates the use of the while statement.

We move the 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 break out of the loop with a break statement.

The True and False are called Boolean types and you can consider them to be equivalent to the value 1 and 0 respectively.

Note for C/C++ Programmers

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

Summary

We have seen how to use the while statement. This is commonly used parts of Python and hence, becoming comfortable with them is essential.