Loops (For, While) – Quick Study Notes
(for the NCO Cyber Olympiad – Software Applications & Programming)
1. What Is a Loop?
- A loop tells the computer to do the same thing again and again.
- Think of it like a chore list: “Wash dishes, then wash dishes, then wash dishes…” until you have finished.
2. Why Use Loops?
- Saves time: you don’t write the same line over and over.
- Makes your program flexible: the number of repeats can change based on input.
3. Two Main Types of Loops
| Type | When to Use | Simple Analogy |
|---|---|---|
| For Loop | When you know exactly how many times you want to repeat. | “Walk 10 steps.” |
| While Loop | When you want to repeat until something happens. | “Keep rinsing until the water is clear.” |
4. The For Loop
How It Looks
for i in range(5): # 5 times
print(i)
What Happens
istarts at 0.- The code inside the loop runs.
imoves to 1, 2, 3, 4.- After 5, the loop stops.
Everyday Analogy
- Counting to 5:
- “Say 1, say 2, say 3, say 4, say 5.”
- Then stop.
Tiny Example – Print Numbers 1‑5
for num in range(1, 6):
print(num)
5. The While Loop
How It Looks
count = 0
while count < 5:
print(count)
count += 1
What Happens
- Check the condition (
count < 5). - If true, run the code inside.
- Update
count. - Go back to step 1.
- Stop when the condition is false.
Everyday Analogy
- Rinse until clean:
- “If the dish is still dirty, rinse it, then check again.”
- Stop when it’s clean.
Tiny Example – Print Numbers 0‑4
x = 0
while x < 5:
print(x)
x += 1
6. Choosing Between For and While
| Situation | Prefer For | Prefer While |
|---|---|---|
| You know how many times (e.g., 10 loops) | ✅ | ❌ |
| You need to keep going until a condition changes (e.g., input > 0) | ❌ | ✅ |
7. Things to Remember
- Always change something inside the loop (like
i += 1orcount += 1).- If you don’t, the loop will never end – an infinite loop.
- Keep the loop body small and clear.
- Think of loops like a “do‑something‑again” command.
8. Common Mistakes
| Mistake | Fix |
|---|---|
| Forgetting to update the counter | Add i += 1 or count += 1 inside the loop. |
| Wrong condition | Double‑check the comparison (<, <=, >, >=). |
Mixing up for and while |
Use for when you know the exact count, while when you don’t. |
9. Quick Practice
- For Loop – Print the letters of the word “HELLO”.
- While Loop – Keep asking the user for a number until they type
0.
(Try writing these on your own before looking at the solutions.)
10. Summary
- Loops repeat actions.
- For – repeat a fixed number of times.
- While – repeat until something is no longer true.
- Always update the loop variable; otherwise, you’ll trap the computer in a never‑ending cycle.
Happy looping! 🚀