Python while Loop

The while loop in Python is used to repeatedly execute a block of code as long as a given condition is True. It’s ideal when you don’t know in advance how many times the loop should run.

Basic Syntax

while condition:
    # Code block to execute repeatedly

• condition: A Boolean expression that evaluates to True or False.
• The loop runs as long as the condition evaluates to True. It stops once the condition becomes False.

Example: Simple Counter

counter = 1
while counter <= 5:
    print("Count:", counter)
    counter += 1  # Increment the counter

Output:

Count: 1
Count: 2
Count: 3
Count: 4
Count: 5

Using a while Loop with User Input

The while loop is commonly used for tasks like validating user input or performing repetitive actions based on user commands.

Example: User Input Validation

number = int(input("Enter a positive number: "))
while number <= 0:
    print("That's not a positive number. Try again!")
    number = int(input("Enter a positive number: "))
print("You entered:", number)

while with Break Condition

You can use the break statement to exit a while loop prematurely when a specific condition is met.

Example: Breaking a Loop

i = 1
while i <= 10:
    if i == 5:
        print("Breaking the loop at", i)
        break
    print(i)
    i += 1

Output:

1
2
3
4
Breaking the loop at 5

Using continue in a while Loop

The continue statement skips the rest of the code in the current iteration and moves to the next iteration.

Example: Skipping a Number

i = 0
while i < 10:
    i += 1
    if i == 5:
        print("Skipping", i)
        continue
    print(i)

Output:

1
2
3
4
Skipping 5
6
7
8
9
10

while-else in Python

The else block in a while loop runs only if the loop condition becomes False naturally (i.e., without encountering a break statement).

Example: Without break

n = 1
while n <= 3:
    print("Inside loop, n =", n)
    n += 1
else:
    print("Loop ended naturally!")

Output:

Inside loop, n = 1
Inside loop, n = 2
Inside loop, n = 3
Loop ended naturally!

Example: With break

n = 1
while n <= 3:
    print("Inside loop, n =", n)
    if n == 2:
        break
    n += 1
else:
    print("Loop ended naturally!")  # This will not execute

Output:

Inside loop, n = 1
Inside loop, n = 2

Advanced Example: Password Checker

Here’s an example of combining a while loop with else for a simple password check program:

password = "pythonforall"
attempts = 3
 
while attempts > 0:
    user_input = input("Enter the password: ")
    if user_input == password:
        print("Access granted!")
        break
    else:
        attempts -= 1
        print("Incorrect password. Attempts left:", attempts)
else:
    print("Too many failed attempts. Access denied.")

Try It Yourself

Problem 1: Multiplication Table

Write a program using a while loop to display the multiplication table of a given number.

num = int(input("Enter a number: "))
i = 1
while i <= 10:
    print(num, "x", i, "=", num * i)
    i += 1

Problem 2: Sum of Digits

Write a program to calculate the sum of digits of a given number using a while loop.

number = int(input("Enter a number: "))
total = 0
while number > 0:
    digit = number % 10
    total += digit
    number //= 10
print("Sum of digits:", total)

Problem 3: Number Guessing Game

Write a program where the user has to guess a randomly generated number. They get unlimited attempts, but the loop stops when they guess correctly.

import random
 
target = random.randint(1, 100)
guess = 0
 
while guess != target:
    guess = int(input("Guess the number (1-100): "))
    if guess < target:
        print("Too low!")
    elif guess > target:
        print("Too high!")
    else:
        print("Correct! The number was", target)

Key Takeaways