Loops in Python — for, while, range, break and continue

A loop repeats code. Instead of writing print 100 times, you write it once inside a loop.

1. The for loop and range()

for i in range(1, 6):
    print(i, end=" ")

1 2 3 4 5

range(start, stop, step):

• range(5) → 0, 1, 2, 3, 4 (starts at 0)
• range(2, 8) → 2 … 7 (stop is not included)
• range(10, 0, -2) → 10, 8, 6, 4, 2 (counting down)

(end=" " keeps printing on the same line instead of a new line.)

Example: sum of 1 to 100

total = 0
for n in range(1, 101):
    total += n
print(total)

5050

2. Looping over a list or string

fruits = ["apple", "mango", "banana"]
for fruit in fruits:
    print(fruit.upper())

for ch in "abc":
    print(ch * 3)

APPLE
MANGO
BANANA
aaa
bbb
ccc

Need the index too? Use enumerate:

for i, fruit in enumerate(["apple", "mango"], start=1):
    print(i, fruit)

1 apple
2 mango

3. The while loop

while repeats as long as a condition is True. Use it when you don't know the number of rounds in advance.

n = 1234
digits = 0
while n > 0:
    n //= 10
    digits += 1
print(digits)

4

Make sure something inside the loop changes the condition, or the loop runs forever (an infinite loop). Press Ctrl + C to stop one.

4. break and continue

• break — leave the loop immediately.
• continue — skip the rest of this round and go to the next.

for n in range(1, 10):
    if n == 6:
        break
    if n % 2 == 0:
        continue
    print(n)

1
3
5

5. Loop else

A loop's else runs only if the loop finished without break. Perfect for "search" loops:

n = 29
for d in range(2, int(n ** 0.5) + 1):
    if n % d == 0:
        print(n, "is not prime")
        break
else:
    print(n, "is prime")

29 is prime

6. Nested loops and patterns

A loop inside a loop: the inner loop runs fully for each round of the outer loop.

for row in range(1, 5):
    print("*" * row)

*
**
***
****

for i in range(1, 4):
    for j in range(1, 4):
        print(i * j, end="\t")
    print()

1	2	3
2	4	6
3	6	9

Common mistakes

• Expecting range(1, 10) to include 10.
• Infinite while loops (forgetting to update the variable).
• Changing a list while looping over it — loop over a copy instead (for x in items[:]).

Quick recap

• for x in range(a, b, step) — b is excluded.
• Loop directly over lists and strings; enumerate for index + item.
• while for unknown counts; always update the condition.
• break leaves; continue skips; loop-else runs if no break.