Loops (for and while)

Loops in Python

Introduction

Loops let you execute a block of code repeatedly. Python has two loop types: for (iterating over sequences) and while (condition-based).

For Loops

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

# Iterating over a string
for char in "Hello":
    print(char)

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

for i in range(2, 10, 3):  # 2, 5, 8
    print(i)

While Loops

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

# Infinite loop (use with caution!)
while True:
    user_input = input("> ")
    if user_input == "quit":
        break

Break and Continue

# break — exit 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

# else runs if loop completed normally (not via break)
for n in range(2, 10):
    for divisor in range(2, n):
        if n % divisor == 0:
            print(f"{n} is not prime")
            break
    else:
        print(f"{n} is prime!")

enumerate() — Indexed Loops

fruits = ["apple", "banana", "cherry"]
for i, fruit in enumerate(fruits, start=1):
    print(f"{i}. {fruit}")
# 1. apple
# 2. banana
# 3. cherry

zip() — Parallel Iteration

names = ["Alice", "Bob", "Charlie"]
scores = [85, 92, 78]
grades = ["B", "A", "C"]

for name, score, grade in zip(names, scores, grades):
    print(f"{name}: {score} ({grade})")

Best Practices

Prefer for over while when iterating over sequences
Use enumerate() instead of manual index tracking
Use zip() for parallel iteration instead of indexing
Avoid modifying a list while iterating — iterate over a copy instead
Use else on loops sparingly — it’s powerful but can be confusing

Practice Exercises

Exercise 1: Print a multiplication table (1-10) using nested loops.

Exercise 2: Find all prime numbers between 1 and 100.

Exercise 3: Implement a simple text-based menu that keeps running until the user chooses to exit.