CyberStars

While Loops

In the previous lesson we learned what a for loop is. We can make the computer do things multiple times automatically. Still, wouldn’t it also be useful to tell the computer to do something as long as… something? Well yes, that’s possible. This is where the while loop comes in

i = 0
while i < 10:
    print(i)
    i = i + 1

this code will display

0
1
2
3
4
5
6
7
8
9

Why not 10 as well? Because if i = 10, then i < 10 is NOT true, since 10 is NOT less than 10

i = 0
while i <= 10:
    print(i)
    i = i + 1

As a matter of fact we will have all the numbers from 0 to 10 inclusive, because 10 <= 10 is true

In other words, while runs as long as the condition is true

Careful! If we don’t write i = i + 1, we enter an infinite loop

i = 0
while i <= 100:
    print(i)

Because i stays 0, therefore print(i) will always print 0. Since i <= 100 is always true in this case (0 <= 100 always), the program will print

0
0
0
0
0
...

forever. This is called an infinite loop

Stopping the loop with input()

The while loop becomes truly powerful together with input(). We can ask the user for something again and again, until they give us the answer we're waiting for

Imagine the station asks for a password. We want to keep asking as long as the password is wrong

password = ""
while password != "starlet":
    password = input("Password: ")
print("Access granted!")

Let's follow what happens:

  • at the start password = "" (empty), so password != "starlet" is true → we enter the loop
  • the program asks for the password. If we type moon, then password = "moon", still different from "starlet" → the loop repeats and asks again
  • if we type starlet, then password = "starlet", so password != "starlet" becomes false → the loop stops
  • the program prints Access granted!

Notice the important thing: here we did not know how many times the loop would repeat. It depends entirely on what the user types. With a for loop it would have been hard, because for wants to know up front how many times to go. With while it's natural: we simply repeat as long as the condition is true

Why did we put password = "" before the loop? So that while has something to check on the first pass. If the variable didn't exist at all, Python would give an error when it reaches the condition

Mission: Access Code

The station asks for an access code before it opens the door. Write a program that repeatedly asks for a code with input(), as long as the typed code is not the correct one. The correct code is 1234

  • as long as the user types a wrong code → print Wrong code
  • when the user types the correct code → print Access granted and the loop stops

Example

If the user types 1111, then 2222, then 1234 in turn, the program prints

Wrong code
Wrong code
Access granted