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31. while Loops

1. Introduction​

A while loop is used to repeat a block of code as long as a condition is true.
It is useful when you don’t know in advance how many times to repeat an action.

2. Basic while Loop​

count = 1

while count <= 5:
    print("Count:", count)
    count += 1

Output:

Count: 1
Count: 2
Count: 3
Count: 4
Count: 5

3. Infinite Loops​

If the condition never becomes false, the loop runs forever.

while True:
    print("This will run forever")

⚠️ Stop infinite loops with Ctrl + C in terminal.

4. Using a Break Condition​

Always ensure there’s a way for the loop to end.

x = 0
while x < 10:
    if x == 5:
        break
    print(x)
    x += 1

5. Using else with while​

The else block runs when the loop ends normally (not by break).

x = 1
while x <= 3:
    print(x)
    x += 1
else:
    print("Loop finished successfully")

Output:

1
2
3
Loop finished successfully

6. Nested while Loops​

You can put a while loop inside another while loop.

i = 1
while i <= 2:
    j = 1
    while j <= 3:
        print(f"i={i}, j={j}")
        j += 1
    i += 1

7. Common Mistakes​

  • Forgetting to update the loop variable → infinite loop.
  • Using = instead of == in the condition.
  • Forgetting the colon (:) after the condition.

8. Practical Examples​

  • Countdown timer:
n = 5
while n > 0:
    print(n)
    n -= 1
print("Blast off!")
  • Input validation:
password = ""
while password != "python123":
    password = input("Enter password: ")
print("Access granted")

9. When to Use while Loops​

  • When the number of iterations is unknown ahead of time.
  • For waiting on user input, file availability, or network events.
  • For loops that depend on conditions instead of a fixed range.

10. Next Steps​

✅ You now know how to use while loops, avoid infinite loops, and apply break/else.
In the next chapter, we’ll explore for loops, which are often used when the number of iterations is known.