Lesson 18: While Loops - Python Course

While Loops

While loops allow you to execute a block of code repeatedly as long as a condition is true. They're perfect for situations where you don't know in advance how many times you need to loop.

Unlike for loops, while loops continue until a condition becomes false, making them ideal for user input validation, game loops, and processing data until a certain condition is met.

Basic While Loop

The simplest while loop executes code while a condition is true. Make sure the condition eventually becomes false, or you'll create an infinite loop:

while.py
count = 0
while count < 5:
    print(count)
    count += 1  # Important: increment to avoid infinite loop

# Output: 0, 1, 2, 3, 4

Loop Control: break and continue

You can control loop execution with break (exit loop) and continue (skip to next iteration):

control.py
count = 0
while count < 10:
    count += 1
    if count == 3:
        continue  # Skip iteration when count is 3
    if count == 7:
        break     # Exit loop when count is 7
    print(count)

# Output: 1, 2, 4, 5, 6

Infinite Loops

An infinite loop runs forever (or until manually stopped). Use them carefully with break to exit:

infinite.py
# Pattern for user input validation
while True:
    user_input = input("Enter 'quit' to exit: ")
    if user_input.lower() == "quit":
        break
    print(f"You entered: {user_input}")

print("Goodbye!")

While Loop with Condition

Common patterns for while loops include counting, checking status, and processing until a condition is met:

patterns.py
# Countdown
timer = 5
while timer > 0:
    print(f"Time remaining: {timer}")
    timer -= 1
print("Time's up!")

# Process until condition is met
number = 10
while number % 2 == 0:
    print(f"{number} is even")
    number += 1
print(f"{number} is odd")

Nested While Loops

You can nest while loops inside other while loops for complex patterns:

nested.py
# Print multiplication table
i = 1
while i <= 3:
    j = 1
    while j <= 3:
        print(f"{i} × {j} = {i * j}", end="  ")
        j += 1
    print()  # New line
    i += 1

While vs For Loops

💡 Use While Loops When:

• You don't know how many iterations you need

• You need to loop until a condition changes

• You're waiting for user input or external events

💡 Use For Loops When:

• You know how many times to iterate

• You're iterating over a collection (list, string, etc.)

• You have a specific range to cover

Common Pitfalls

⚠️ Infinite Loops

Always ensure your condition will eventually become false. If your loop variable doesn't change, you'll have an infinite loop that never stops!

⚠️ Off-by-One Errors

Be careful with your loop conditions. while count < 5 runs 5 times (0-4), but while count <= 5 runs 6 times (0-5).

While Loops

While Loops

Basic While Loop

Loop Control: break and continue

Infinite Loops

While Loop with Condition

Nested While Loops

While vs For Loops

💡 Use While Loops When:

💡 Use For Loops When:

Common Pitfalls

⚠️ Infinite Loops

⚠️ Off-by-One Errors

Lesson Complete!