🌱

Basic

for · while · range · break · continue · else

Week 5 — Loops

Automate repetitive tasks with for and while loops. Master range(), break, continue, and the rarely-known loop else clause.

forwhilerangebreakcontinueloops

Duration

⏱ 2.5 hours

Level

📊 Beginner

Prerequisite

🎯 Week 4

OUTCOME

Build a multiplication table generator and a number guessing game

What you'll learn

1Iterate over a range with for/range()
2Use while for condition-driven loops
3Control loop flow with break and continue
4Explain the loop else clause
5Nest loops to produce 2D output

1. for Loop & range()

python

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

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

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

ℹ️

range(stop), range(start, stop), range(start, stop, step). The stop value is exclusive.

2. while Loop

python

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

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

3. break, continue, else

python

for i in range(1, 11):
    if i == 5:
        continue   # skip 5
    if i == 8:
        break      # stop at 8
    print(i, end=' ')  # 1 2 3 4 6 7

for i in range(2, 10):
    for j in range(2, i):
        if i % j == 0:
            break
    else:           # only runs if no break
        print(f"{i} is prime")

4. Nested Loops

python

for row in range(1, 4):
    for col in range(1, 4):
        print(f"{row}×{col}={row*col}", end='  ')
    print()

Output: 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

5. Common Mistakes

Forgetting to increment the counter in a while loop → infinite loop. Always update the condition variable.
range(1, 10) ends at 9, not 10. Off-by-one errors are very common.
Modifying a list while iterating over it — iterate over a copy instead: for x in lst[:].

💻 Examples

Run these examples and check the output yourself.

01_times_table.py— Multiplication table 1–9

CODE

for i in range(1, 10):
    for j in range(1, 10):
        print(f"{i*j:3d}", end='')
    print()

02_sum_1_to_n.py— Sum 1 to N using for and while

CODE

n = int(input("N: "))
# for version
total = sum(range(1, n+1))
print(f"for: {total}")
# while version
total, i = 0, 1
while i <= n:
    total += i
    i += 1
print(f"while: {total}")

▶ Output

N: 100
for: 5050
while: 5050

03_guess_number.py— Number guessing game with limited attempts

CODE

import random
secret = random.randint(1, 100)
attempts = 0
while True:
    guess = int(input("Guess (1-100): "))
    attempts += 1
    if guess < secret:
        print("Too low!")
    elif guess > secret:
        print("Too high!")
    else:
        print(f"Correct! It took {attempts} attempts.")
        break

📝 Exercises

Try them yourself first, then open the solution to compare.

Exercise 1

FizzBuzz

Goal: Print 1 to 100, replacing multiples of 3 with Fizz, 5 with Buzz, 15 with FizzBuzz.

Requirements

Use a for loop with range(1, 101)
Check divisibility with %
Print FizzBuzz for multiples of 15 first

▶Toggle solution

SOLUTION

for i in range(1, 101):
    if i % 15 == 0:
        print('FizzBuzz')
    elif i % 3 == 0:
        print('Fizz')
    elif i % 5 == 0:
        print('Buzz')
    else:
        print(i)

Exercise 2

Star Pyramid

Goal: Print a right-aligned pyramid of stars.

Requirements

Read height N from input
Row k prints (N-k) spaces then k stars
Use nested loops

Sample I/O

N: 5
    *
   **
  ***
 **** 
*****

▶Toggle solution

SOLUTION

n = int(input('N: '))
for i in range(1, n+1):
    print(' ' * (n-i) + '*' * i)

Example code / lecture materials

All lecture materials and example code are openly available on GitHub.

View on GitHub ↗