Lesson 05 · Loops¶

The for loop¶

for repeats a block of code for each element in a sequence:

for i in range(5):
    print(i)

Output:

0
1
2
3
4

The range() function¶

# Count from 1 to 10
for i in range(1, 11):
    print(i)

# Even numbers from 2 to 20
for i in range(2, 21, 2):
    print(i)

# Count down
for i in range(5, 0, -1):
    print(i)
print("Start!")

for over a string¶

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

The while loop¶

while repeats a block as long as a condition is true:

number = 1
while number <= 5:
    print(number)
    number += 1   # number = number + 1

Output:

1
2
3
4
5

# WRONG — infinite loop!
x = 1
while x <= 5:
    print(x)
    # forgot x += 1

while for input validation¶

answer = ""
while answer != "yes" and answer != "no":
    answer = input("Did you understand? (yes/no): ")
print("Thank you!")

break — forced exit from the loop¶

for i in range(1, 100):
    if i == 5:
        break
    print(i)
# Prints: 1, 2, 3, 4

continue — skip to the next iteration¶

for i in range(1, 11):
    if i % 2 == 0:
        continue   # skip even numbers
    print(i)
# Prints: 1, 3, 5, 7, 9

Nested loops¶

A loop inside another loop:

for i in range(1, 4):
    for j in range(1, 4):
        print(f"{i} × {j} = {i * j}")
    print()

Output:

1 × 1 = 1
1 × 2 = 2
1 × 3 = 3

2 × 1 = 2
...

Exercises¶

Exercise 1 — Sum from 1 to N¶

Ask for a number N and calculate the sum 1 + 2 + ... + N.

n = int(input("N = "))
total = 0
for i in range(1, n + 1):
    total += i
print(f"Sum = {total}")

Exercise 2 — Prime numbers¶

Display all prime numbers smaller than 50.

for n in range(2, 50):
    is_prime = True
    for d in range(2, n):
        if n % d == 0:
            is_prime = False
            break
    if is_prime:
        print(n)

Exercise 3 — Guess the number¶

Generate a "secret" number (hardcoded) and let the user guess with feedback.

secret = 42
attempts = 0

while True:
    guess = int(input("Guess the number (1-100): "))
    attempts += 1
    if guess < secret:
        print("Too low!")
    elif guess > secret:
        print("Too high!")
    else:
        print(f"Correct! You guessed in {attempts} attempts.")
        break

Mini-project: Multiplication table¶

Display the multiplication table for numbers 1–10 in a tabular format.

Output (first rows):

   1   2   3   4   5   6   7   8   9  10
   2   4   6   8  10  12  14  16  18  20
   3   6   9  12  15  18  21  24  27  30
...

for i in range(1, 11):
    for j in range(1, 11):
        print(f"{i * j:4}", end="")
    print()

Summary¶

• for i in range(n): — repeat n times
• range(start, stop, step) — generate sequences of numbers
• while condition: — repeat as long as the condition is true
• break — forced exit from the loop
• continue — skip to the next iteration
• Indentation is mandatory!

Next step: → Lesson 06: Lists