Python Basics — While Loop

While Loop

A while loop repeats as long as a condition is True. Unlike for loops that iterate over a known sequence, while loops are for situations where you don’t know in advance how many iterations you’ll need.

Basic Syntax

while condition:
    # code to repeat

count = 1

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

Output:

1
2
3
4
5

How it works:

1. Check condition (count <= 5)
2. If True, run the body
3. Go back to step 1
4. If False, exit the loop

Infinite Loops (and How to Avoid Them)

If the condition never becomes False, the loop runs forever:

# WARNING: Infinite loop! (Don't run this)
# while True:
#     print("This never stops!")

Common causes:

• Forgetting to update the loop variable
• Condition that can never become False
• Logic errors

# Bug: forgot to increment count
count = 1
# while count <= 5:
#     print(count)
#     # Missing: count += 1  → infinite loop!

Intentional infinite loop with break:

while True:
    user_input = input("Type 'quit' to exit: ")
    if user_input == "quit":
        print("Goodbye!")
        break
    print(f"You typed: {user_input}")

Counter-Controlled While Loops

When you know how many times to repeat but need more control:

# Countdown
countdown = 5

while countdown > 0:
    print(f"{countdown}...")
    countdown -= 1

print("Liftoff!")

Output:

5...
4...
3...
2...
1...
Liftoff!

while-else

Like for-else, the else block runs when the condition becomes False (not via break):

attempts = 0
password = "secret"

while attempts < 3:
    guess = input("Enter password: ")
    if guess == password:
        print("Access granted!")
        break
    attempts += 1
    print(f"Wrong! {3 - attempts} attempts remaining.")
else:
    print("Account locked! Too many failed attempts.")

If all 3 attempts fail, else runs. If the correct password is entered, break skips the else.

Input Validation Pattern

One of the most common uses of while:

while True:
    try:
        age = int(input("Enter your age: "))
        if age < 0 or age > 150:
            print("Please enter a realistic age.")
            continue
        break
    except ValueError:
        print("Invalid input. Please enter a number.")

print(f"Your age: {age}")

Menu-Driven Program

while True:
    print("\n--- Menu ---")
    print("1. Say Hello")
    print("2. Say Goodbye")
    print("3. Exit")

    choice = input("Choose (1-3): ")

    if choice == "1":
        print("Hello, World!")
    elif choice == "2":
        print("Goodbye, World!")
    elif choice == "3":
        print("Exiting...")
        break
    else:
        print("Invalid choice. Try again.")

for vs while

Feature	for	while
Use when	Iterating over a sequence	Repeating until a condition changes
Iterations	Known in advance	Unknown / dynamic
Common use	Processing items	Input validation, game loops
Risk	Rarely infinite	Can be infinite
Typical	for item in list:	while condition:

Rule of thumb: If you’re iterating over something, use for. If you’re waiting for something, use while.

Practical Examples

Sum Until Negative

total = 0

while True:
    num = float(input("Enter a number (negative to stop): "))
    if num < 0:
        break
    total += num

print(f"Total: {total}")

Finding a Number

import random

target = random.randint(1, 100)
guesses = 0

while True:
    guess = int(input("Guess (1-100): "))
    guesses += 1

    if guess < target:
        print("Too low!")
    elif guess > target:
        print("Too high!")
    else:
        print(f"Correct! You got it in {guesses} guesses.")
        break

Summary

• while condition: repeats as long as the condition is True
• Always ensure the condition will eventually become False
• Use break for controlled exit from intentional infinite loops
• while-else runs the else block when the condition becomes False
• Use while True + break for input validation patterns
• Prefer for when iterating over sequences; use while when waiting for a condition

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