Loops: for and while

The for Loop

Iterates over any iterable (list, string, range, dict, etc.):

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

# Iterate over a string
for char in "Python":
    print(char)

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

for i in range(2, 10, 2):  # start, stop, step
    print(i)               # 2, 4, 6, 8

range() in Detail

range(5)         # 0 to 4
range(1, 6)      # 1 to 5
range(0, 10, 2)  # 0, 2, 4, 6, 8
range(5, 0, -1)  # 5, 4, 3, 2, 1 (countdown)

# Convert to list to see values:
print(list(range(5)))  # [0, 1, 2, 3, 4]

The while Loop

Runs while a condition is True:

count = 0
while count < 5:
    print(count)
    count += 1

# Infinite loop with break
while True:
    user_input = input("Enter 'quit' to exit: ")
    if user_input == "quit":
        break
    print(f"You entered: {user_input}")

break and continue

# break — exit the loop entirely
for i in range(10):
    if i == 5:
        break
    print(i)  # 0, 1, 2, 3, 4

# continue — skip to next iteration
for i in range(10):
    if i % 2 == 0:
        continue
    print(i)  # 1, 3, 5, 7, 9

else Clause on Loops

The else block runs if the loop completed without hitting break:

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

for n in numbers:
    if n == target:
        print("Found!")
        break
else:
    print("Not found")   # runs because break was never hit

Nested Loops

# Multiplication table
for i in range(1, 4):
    for j in range(1, 4):
        print(f"{i}×{j}={i*j}", end="  ")
    print()
# 1×1=1  1×2=2  1×3=3
# 2×1=2  2×2=4  2×3=6
# 3×1=3  3×2=6  3×3=9

Useful Iteration Patterns

# enumerate — get index and value
fruits = ["apple", "banana", "cherry"]
for i, fruit in enumerate(fruits):
    print(f"{i}: {fruit}")

# enumerate with start index
for i, fruit in enumerate(fruits, start=1):
    print(f"{i}. {fruit}")

# zip — iterate multiple iterables together
names = ["Alice", "Bob", "Charlie"]
scores = [95, 87, 92]
for name, score in zip(names, scores):
    print(f"{name}: {score}")

# zip_longest from itertools:
from itertools import zip_longest
for a, b in zip_longest([1, 2, 3], ["a", "b"], fillvalue=None):
    print(a, b)

Exercises

Exercise 1: Sum of Squares

Calculate the sum of squares of numbers 1 to 10.

Solution:

total = 0
for i in range(1, 11):
    total += i ** 2
print(total)  # 385

Exercise 2: Countdown with while

Print a countdown from 10 to 1, then print "Blast off!"

Solution:

n = 10
while n > 0:
    print(n)
    n -= 1
print("Blast off!")

Exercise 3: Prime Numbers

Find all prime numbers between 2 and 50 using nested loops.

Solution:

primes = []
for n in range(2, 51):
    for divisor in range(2, int(n**0.5) + 1):
        if n % divisor == 0:
            break
    else:
        primes.append(n)
print(primes)
# [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47]

Exercise 4: Pattern Printing

Print this triangle pattern using nested loops:

*
**
***
****
*****

Solution:

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

Exercise 5: Digit Sum

Find the sum of digits of the number 9876543210 using a loop.

Solution:

number = 9876543210
digit_sum = sum(int(d) for d in str(number))
print(digit_sum)  # 45

# Alternative with while:
n = 9876543210
total = 0
while n > 0:
    total += n % 10
    n //= 10
print(total)  # 45