While Loops

While

The structure of a while loop is similar to a for loop but instead of labelling a variable and specifying a numerical range we specify a condition like we did with the if statements. The while loop will keep repeating as long as this condition is true.

again = "yes"

while again == "yes":

    print("You are a wonderful human being")

    again = input("Would you like to hear that again ? yes/no ")

print("Ok, goodbye")

We have to set our variable up before the while loop so it can do the first loop and get to the input line. We don't need to do this with a for loop as the variable is created in the first line of the for loop. This program will keep printing its message until the user types in something other than yes. 

Booleans

We have another data type in Python called a Boolean. Named after George Boole, a clever bloke who did a lot of work on logic around 1850. Booleans can only be either True or False. This makes them ideal for controlling while loops as the program checks the condition and if it is True it does the loop, if it is False then it skips the loop and carries on with the program. If we set a variable to True then we don't even have to check if the variable == True we can just use the variable label. eg

running = True

while running:

    print("This program is running fine")

This program will keep printing its message in the Python shell for ever as we haven't programmed a way for running to become False. This would be better:

running = True

while running:

    print("This program is running fine")

    again = input("Do you want the program to keep running ?  (y/n) ")

    if again == "n":

        running = False

print("OK, I'll stop then")

This program will keep printing its message until the user enters 'n', then the variable running becomes False so when it gets checked at the start of the next loop it is not True and the loop skips to the print("OK, I'll stop then") line. If anything else is entered by the user - 'yes', 'y' or just pressing the enter key - the variable running doesn't get changed, so when it is checked at the start of the next loop it is still True and the message is printed once more.

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