Loop Conditions

Loop Conditions

In this tutorial, we'll control exactly when a while loop continues.

The condition controls how long a while loop continues.

lives = 3

while lives > 0:
    print("Keep playing")
    lives = lives - 1

The loop runs while lives > 0 is true.

Check Before Running

A while loop checks the condition before the loop body runs.

count = 10

while count < 5:
    print("This will not print")

The condition is false at the start, so the body never runs.

Off-by-One Errors

Be careful with < and <=.

count = 1

while count < 5:
    print(count)
    count = count + 1

This prints 1 to 4, not 1 to 5.

To include 5:

while count <= 5:
    print(count)
    count = count + 1

Combining Conditions

You can use and or or:

attempts = 0
password = ""

while password != "secret" and attempts < 3:
    password = input("Password: ")
    attempts = attempts + 1

This loop stops when either the password is correct or the attempts reach 3. When conditions are combined, read them slowly in plain English.

Print the Condition Values

If a loop surprises you, print the values used in the condition:

print("attempts:", attempts)
print("password:", password)

This usually reveals why the loop continued or stopped.

Practice

Write a loop that counts from 2 to 10, including 10.