while Loop

The while loop repeats a block as long as the condition remains True. Used when the number of iterations is not known in advance.

Syntax

while condition:
    statement(s)

i = 1
while i <= 5:
    print(i)
    i += 1

Output:

1
2
3
4
5

Important: the loop variable must be updated inside the body, or the loop never terminates (infinite loop).

# Infinite loop example (do NOT run without a break condition)
# while True:
#     print("runs forever")

while-else

n = 0
while n < 3:
    print(n)
    n += 1
else:
    print("Loop ended normally (no break)")

Example: Sum of digits using while

num = 1234
total = 0
while num > 0:
    total += num % 10
    num //= 10
print("Sum of digits:", total)   # 10

Example: Reverse a number

num = 1234
reverse = 0
while num > 0:
    digit = num % 10
    reverse = reverse * 10 + digit
    num //= 10
print("Reversed:", reverse)   # 4321

for vs while — when to use which