While loops

To be useful any programming language must allow you to repeat a portion of a program more than once. These types of repetitions are called loops. There are two kinds of loops in Python. Below, we will discuss the first of them: the while loop. It is used to repeat a given block as long as a certain condition is met. This loop looks like this:

while condition:
    block of statements executed
    as long as the condition
    is True

Please note the colon after the condition and the block indentation — the same rules for defining blocks apply as for the if statement. Sample code:

number = 0
while number < 1 or number > 10:
     number = int(input("Enter an integer from 1 to 10:"))
print("Your number is", number)

In the above example, the user will be prompted to enter a number as long as the value of the variable number to which the value is assigned is either less than 1 or greater than 10.

The condition in the while loop is checked at the very beginning, before the loop is executed for the first time. This means that the loop may not be executed even once if the condition is not met at the very beginning. Please note the first line of the example above number = 0. It defines a variable number and sets its value to zero, which guarantees that the first time it checks, the while loop will be true (0 < 1) i.e. the user will be prompted for a number at least once.

Breaking the loop

The standard termination of the while loop occurs when the condition has a value False. When set to value True, the entire loop block is executed. This is not always what you want. Therefore, it is possible to interrupt execution of a loop inside its block with an instruction break. Let’s see an example of a primitive version of the 21 game:

sum = 0 
given_number = int(input("Enter a number: ")) 
while given_number != 0:
    sum += given_number   # this entry is a shortened version of the assignment sum + = sum + given_number
    if sum > 21:
        print("You have exceeded 21 and you lost! ")
        sum = 0
        break
    given_number = int(input("Enter a number (0 to end): ")) 
print("Your score is", sum)

Normal loop break occurs when the user enters the number 0 (the condition will not be fulfilled then given_number != 0). However, when the calculated sum exceeds 21, the loop is stopped abnormally using the break command. In both cases, the program will continue from the first command outside the loop block (print("Your score is", sum)).

After a completed loop block, it is possible to add a command else followed by a block, which will only be executed if the loop ended normally (that is, not via a command break). Using this, you can write the above example in a slightly more elegant way:

sum = 0
given_number = int(input("Enter a number: "))
while given_number != 0:
    sum += given_number
    if sum > 21:
        print("You've exceeded 21 and lost!")
        break
    given_number = int(input("Enter number (0 to finish): ")) 
else:
    print("You won! Your score is", sum)

The last line will only be printed when the loop ends with a zero, not by exceeding 21.

Continuation of the loop

Another statement that is used to control loop execution is continue. If the Python interpreter hits continue somewhere in the middle of a loop iteration, it skips all remaining statements and proceeds to the next iteration. Example:

i = -11
while i < 10:
    i += 1
    if i == 0:
        continue
    print(1 / i)

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