While Loops, Loop Until Done

Use while loops when you don't know how many times to repeat

A `for` loop runs a known number of times. A `while` loop runs as long as a condition is true, you use it when you don't know in advance how many iterations you need.

How would you approach this? Think first, then expand.

While loop basicspython

count = 1
while count <= 5:
    print(count)
    count += 1
# prints: 1, 2, 3, 4, 5

🔍Code TracerStep 1 / 14

trace.py

1count = 0←

2while count < 3:

3 print(count)

4 count += 1

5print("done")

Variables

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Step 1 / 14

Infinite loop danger! If the condition never becomes False, your program runs forever. Always make sure something inside the loop changes the condition. count += 1 is what stops the loop above.

Breaking out of a looppython

# Find first number divisible by 7
n = 1
while True:           # runs forever... unless we break
    if n % 7 == 0:
        print(f"First multiple of 7: {n}")
        break         # exit the loop immediately
    n += 1
# prints: First multiple of 7: 7

When to use which:

for loop → you know the count, or you're iterating over a list

while loop → you're waiting for a condition to change (searching, guessing games, retries)

Practice Exercises

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Exercise 1 of 2easy

⏱ 00:00

Countdown

Write a function countdown(n) that prints numbers from n down to 1, then prints "Go!".
Example for countdown(5):`54321Go!`

solution.py

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Exercise 2 of 2medium

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Digit Sum

Write a function digit_sum(n) that returns the sum of all digits of a positive integer.
Example: digit_sum(1234) → 10 (1+2+3+4)

solution.py

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Solve all 2 exercises to unlock completion

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