While

In addition to the for loop, we have a second type of loop, namely, the while loop. Unlike for, where we know the number of repetitions in advance, while is used when the loop depends on some condition. The loop body is repeated until the condition is met.

response = input('Say aaa!')
while response != 'aaa':
    print('Incorrect, try again')
    response = input('Say aaa!')

But pay attention! It is very easy to write a loop with a condition that is always true. This kind of loop will be repeated forever.

from random import randrange

while True:
    print('The number is', randrange(10000))
    print('(Wait for the computer to get tired…)')

The program can be interrupted with Ctrl+C.

Finally, we have the break command, that will signal the process to ‘jump out’ of the loop, and commands after the loop will be performed immediately.

while True:
    response = input('Say aaa!')
    if response == 'aaa':
        print('Good')
        break
    print('Incorrect, try again')

print('Yay and it did not even hurt')

The break command can only be used inside a loop (while or for), and if we have nested loops, it will only jump out of the one where it is used.

for i in range(10):  # Outer loop
    for j in range(10):  # Inner loop
        print(j * i, end=' ')
        if i <= j:
            break
    print()

Break will jump out of the inner loop and back to the outer loop when i is less or equal than j.

Back to while! Can you write the following program?

21

• You start with 0 points
• In each round, the computer shows how many points you have, and asks if you want to continue
• If you answer ‘No’, the game ends.
• If you answer ‘Yes’, the computer ‘turns a card’ (randomly selects a number from 2 to 10) and adds its value to the current points.
• If you go over 21, you lose
• The aim of the game is to get as many points as possible, but you win only if you get 21.

from  random  import  randrange

sum = 0
while sum < 21:
    print('You have', sum, 'points.')
    answer = input('Turn card? ')
    if answer == 'yes':
        card = randrange(2,11)
        print("You've got number", card)
        sum = sum + card
    elif answer == 'no':
        break
    else:
        print('I do not understand! Answer "yes" or "no"' )

if sum == 21:
    print('Congratulations! You won!')
elif sum > 21:
    print('Bad luck!', sum, 'points is too much!')
else:
    print('So close! You missed', 21 - sum, 'points!')