Loops

What you'll learn

How to repeat code with for and while loops and generate numbers with range().

Why it matters

Looping over names, adding up prices or retrying until something works are everyday tasks. A loop lets you write the instruction once.

Explanation

A for loop goes through a sequence, one item at a time. range(n) produces the numbers 0 up to (but not including) n.

range(start, stop) begins at start: range(1, 4) gives 1, 2, 3.

A while loop keeps going as long as its condition is True. Make sure something inside the loop eventually makes the condition False, or it never stops.

break leaves a loop early and continue skips to the next round.

Example

Step-by-step walkthrough

1. range(1, 4) yields 1, 2, 3 so the first loop prints Round 1, Round 2, Round 3.
2. The while loop adds n to total each round: 1, 3, 6, 10, 15, then n becomes 6 and the loop ends.
3. It prints Sum 1 to 5 is 15.
4. The last loop visits each fruit and prints it upper-cased: APPLE, BANANA, CHERRY.

Try it yourself

Change the first loop to count from 1 to 10 and print only the even numbers. Hint: i % 2 == 0.

LoopsPython

Editable codefor i in range(1, 4): print("Round", i) for fruit in ["apple", "banana", "cherry"]: print(fruit.upper())

Run ResetCopyCtrl+Enter to run

Common mistakes

• Off by one: range(5) stops at 4, not 5.
• Forgetting to update the counter in a while loop, causing an infinite loop.
• Modifying a list while looping over it. Build a new list instead.
• Missing colon or indentation after for / while.

Quick quiz

Summary

• for item in sequence: visits each item; range() makes number sequences.
• while condition: repeats until the condition is False.
• break and continue control the flow inside loops.

You finished the Python beginner section. Next: functions, lists and dictionaries.

for i in range(3):
    print(i)    # 0, 1, 2

for i in range(1, 4):
    print("Round", i)

total = 0
n = 1
while n <= 5:
    total = total + n
    n = n + 1
print("Sum 1 to 5 is", total)

for fruit in ["apple", "banana", "cherry"]:
    print(fruit.upper())