Loops: for and while

Loops let you repeat a block of code multiple times without writing it out over and over. Python provides two main loop types: for and while.

The for Loop

A for loop iterates over a sequence — a list, string, range, or any iterable:

fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)
# apple
# banana
# cherry

Looping with range()

range() generates a sequence of numbers:

for i in range(5):
    print(i)
# 0 1 2 3 4

for i in range(1, 6):
    print(i)
# 1 2 3 4 5

for i in range(0, 10, 2):
    print(i)
# 0 2 4 6 8

range(start, stop, step) — stop is exclusive, step defaults to 1.

Iterating Over Strings

for char in "Python":
    print(char)
# P y t h o n

The while Loop

A while loop repeats as long as its condition is True:

count = 0
while count < 5:
    print(count)
    count += 1
# 0 1 2 3 4

Always make sure something changes inside a while loop to eventually make the condition False — otherwise you get an infinite loop.

break and continue

break exits the loop immediately:

for i in range(10):
    if i == 5:
        break
    print(i)
# 0 1 2 3 4

continue skips the rest of the current iteration and moves to the next:

for i in range(10):
    if i % 2 == 0:
        continue
    print(i)
# 1 3 5 7 9

The else Clause on Loops

Python loops can have an else block that runs when the loop completes normally (without hitting a break):

for i in range(5):
    print(i)
else:
    print("Loop finished!")

Nested Loops

Loops can be nested inside each other:

for row in range(1, 4):
    for col in range(1, 4):
        print(row * col, end=" ")
    print()
# 1 2 3
# 2 4 6
# 3 6 9

enumerate()

When you need both the index and value while iterating, use enumerate():

colors = ["red", "green", "blue"]
for index, color in enumerate(colors):
    print(f"{index}: {color}")
# 0: red
# 1: green
# 2: blue