Loops in Python

In this lab you'll practice writing loops — one of the most fundamental tools in programming. Loops let you repeat actions without writing the same code over and over.

Background

Python has two main types of loops:

for loops iterate over a sequence (like a range of numbers or characters in a string):

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

while loops repeat as long as a condition is true:

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

Exercise 1: Counting Up

Write a program that prints the numbers from 1 to 10, each on its own line.

Expected output:

1
2
3
...
10

Solution:

for i in range(1, 11):
    print(i)

Exercise 2: Sum of Numbers

Write a program that computes the sum of the numbers from 1 to 100 and prints the result.

Hint: Create a variable to hold the running total before the loop starts.

Expected output:

The sum is 5050

Solution:

total = 0
for i in range(1, 101):
    total += i
print(f"The sum is {total}")

Exercise 3: Multiplication Table

Write a program that prints the multiplication table for a given number. Ask the user for a number, then print its multiples from 1 to 10.

Example output (if user enters 7):

7 x 1 = 7
7 x 2 = 14
7 x 3 = 21
...
7 x 10 = 70

Solution:

num = int(input("Enter a number: "))
for i in range(1, 11):
    print(f"{num} x {i} = {num * i}")

Exercise 4: Input Validation

Write a program that keeps asking the user to enter a password until they type "secret123". When they get it right, print a welcome message. If they get it wrong, tell them to try again.

Example interaction:

Enter password: hello
Incorrect, try again.
Enter password: secret123
Welcome!

Solution:

password = input("Enter password: ")
while password != "secret123":
    print("Incorrect, try again.")
    password = input("Enter password: ")
print("Welcome!")

Exercise 5: Counting Characters

Write a program that asks the user for a word and a letter, then counts how many times that letter appears in the word.

Example output:

Enter a word: banana
Enter a letter: a
The letter 'a' appears 3 times in 'banana'

Solution:

word = input("Enter a word: ")
letter = input("Enter a letter: ")
count = 0
for char in word:
    if char == letter:
        count += 1
print(f"The letter '{letter}' appears {count} times in '{word}'")

Exercise 6: Number Guessing Game

Write a simple number guessing game. The secret number is 42. The program should:

Ask the user to guess a number
Tell them if their guess is too high or too low
Keep looping until they guess correctly
Count and display the number of attempts

Example interaction:

Guess a number: 25
Too low!
Guess a number: 50
Too high!
Guess a number: 42
Correct! You got it in 3 attempts.

Solution:

secret = 42
attempts = 0

guess = int(input("Guess a number: "))
attempts += 1

while guess != secret:
    if guess < secret:
        print("Too low!")
    else:
        print("Too high!")
    guess = int(input("Guess a number: "))
    attempts += 1

print(f"Correct! You got it in {attempts} attempts.")

Wrap-Up

You practiced:

for loops with range() for counting and accumulating
for loops over strings for character processing
while loops for input validation and game logic
The accumulator pattern (building up a result across iterations)

These patterns show up everywhere in programming. Next, try combining loops with lists and functions to solve bigger problems.