Summary of Today’s Python Classroom: Loop Until It’s True – While Loops and Prime Numbers (23 Sep 2026)

Today we moved from conditionals to repetition: the while loop, how it can run forever, and how to use it to check whether a number is prime.

1. While Loop Fundamentals

• A while loop runs a block repeatedly until its condition becomes false.
• The condition is checked before each pass. If it’s false at the start, the block never runs at all.
• Set count = -5 with index = 0 and Python skips the loop entirely, moving straight to the next line. That’s not a bug, it’s the rule.

index = 0
while index < 5:
    print("hello")
    index = index + 1

2. Infinite Loops

• Forget to increment the counter and the condition never turns false. The program runs forever.
• Ctrl + C in the terminal sends the exit signal and stops it.
• Every while loop needs an answer to one question: what makes this condition eventually false?

3. Debugging the Loop

We stepped through the loop in the Python debugger, watching the index change and the condition get re-evaluated on each pass. When a loop misbehaves, don’t stare at the code. Step through it and watch the variables.

4. Prime Number Checker

A prime number is divisible only by 1 and itself.

n = int(input("Enter a number: "))
is_prime = True

i = 2
while i < n:
    if n % i == 0:
        is_prime = False
        break      # stop as soon as a divisor is found
    i = i + 1

print(is_prime)

• input() returns a string, so wrap it in int() before doing arithmetic.
• break exits the loop immediately. Once you’ve found one divisor, the answer is settled and further checks are wasted work.
• We traced it in the debugger with 5 (prime) and 51 (not prime, since 3 divides it).

✅ Exercises

1. Warm-up: sum all multiples of 3 or 5 below 10. The answer is 23, so you can check yourself.
2. Project Euler Problem 1: sum all multiples of 3 or 5 below 1000.
3. Edge cases: run the prime checker with 0, 1 and -7. Watch what the loop does when n is small, then decide what the program should say and fix it.

Stretch (optional): once it works, try checking divisors only up to the square root of n. Same answer, far less work.

Remember: attempting the code matters more than getting it right first go. Write it, run it, break it, fix it.

⏭️ Next up: the continue statement, the companion to break.