Loops: for and while

Loops let you repeat a block of code multiple times. Python has two loop constructs: for loops (for iterating over sequences) and while loops (for repeating while a condition is true). Mastering loops is essential — they are one of the most powerful tools in programming.

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The for Loop

A for loop iterates over a sequence (list, string, range, etc.):

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

# Loop through a string
for char in "Python":
    print(char, end=" ")
# P y t h o n

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The range() Function

range() generates a sequence of numbers — perfect for loops:

# range(stop) — 0 to stop-1
for i in range(5):
    print(i, end=" ")
# 0 1 2 3 4

# range(start, stop)
for i in range(2, 6):
    print(i, end=" ")
# 2 3 4 5

# range(start, stop, step)
for i in range(0, 20, 3):
    print(i, end=" ")
# 0 3 6 9 12 15 18

# Counting backwards
for i in range(10, 0, -1):
    print(i, end=" ")
# 10 9 8 7 6 5 4 3 2 1

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The while Loop

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

count = 0
while count < 5:
    print(f"Count is {count}")
    count += 1
# Count is 0
# Count is 1
# Count is 2
# Count is 3
# Count is 4

Danger: If the condition never becomes False, you get an infinite loop:

# DON'T do this (infinite loop!)
# while True:
#     print("This never stops!")

# But infinite loops with break are a common pattern:
while True:
    answer = input("Type 'quit' to exit: ")
    if answer == "quit":
        break
    print(f"You typed: {answer}")

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break — Exit a Loop Early

break immediately stops the loop:

# Find the first even number
numbers = [1, 3, 7, 8, 12, 15]
for num in numbers:
    if num % 2 == 0:
        print(f"First even number: {num}")
        break
# First even number: 8

# Input validation loop
while True:
    age = input("Enter your age (or 'quit'): ")
    if age == "quit":
        print("Goodbye!")
        break
    if age.isdigit():
        print(f"You are {age} years old.")
        break
    print("Please enter a valid number.")

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continue — Skip to the Next Iteration

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

# Print only odd numbers
for i in range(10):
    if i % 2 == 0:
        continue  # skip even numbers
    print(i, end=" ")
# 1 3 5 7 9

# Skip blank lines when processing text
lines = ["Hello", "", "World", "", "Python"]
for line in lines:
    if not line:
        continue
    print(line)
# Hello
# World
# Python

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else on Loops

Python has a unique feature — an else clause on loops. It runs when the loop completes without hitting a break:

# Search for a number
numbers = [1, 3, 5, 7, 9]
target = 4

for num in numbers:
    if num == target:
        print(f"Found {target}!")
        break
else:
    print(f"{target} not found in the list.")
# 4 not found in the list.

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Enumerate — Loop with Index

enumerate() gives you both the index and the value:

fruits = ["apple", "banana", "cherry"]

# Without enumerate
for i in range(len(fruits)):
    print(f"{i}: {fruits[i]}")