While Loops Basics

What is a While Loop?

A while loop repeats code as long as a condition is True. Unlike for loops that iterate over a sequence, while loops continue until the condition becomes False.

Basic While Loop Syntax

while condition:
    # Code to repeat
    pass

The loop continues as long as condition is True.

Your First While Loop

Here's a simple example:

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

Output:

0
1
2
3
4

The loop runs while count < 5 is True. When count becomes 5, the condition is False and the loop stops.

Understanding the Condition

The condition is checked before each iteration:

number = 1
while number <= 5:
    print(f"Number: {number}")
    number = number + 1

Output:

Number: 1
Number: 2
Number: 3
Number: 4
Number: 5

While vs For Loops

For loops: Use when you know how many times to repeat or when iterating over a collection.

While loops: Use when you want to repeat until a condition changes.

# For loop - we know we want 5 iterations
for i in range(5):
    print(i)

# While loop - repeat until condition is false
count = 0
while count < 5:
    print(count)
    count = count + 1

Both produce the same output, but for loops are usually clearer when you know the number of iterations.

Practical Examples

Example 1: Counting Up

number = 1
while number <= 10:
    print(number)
    number = number + 1

Example 2: User Input Validation

# Simulating user input
password = ""
while password != "secret":
    password = "secret"  # In real code, this would be input()
    if password != "secret":
        print("Wrong password, try again!")
print("Access granted!")

Example 3: Processing Until Done

tasks = ["task1", "task2", "task3"]
while tasks:
    current_task = tasks.pop(0)
    print(f"Processing: {current_task}")
print("All tasks done!")

Output:

Processing: task1
Processing: task2
Processing: task3
All tasks done!

Common Patterns

Pattern 1: Counter Pattern

count = 0
while count < 5:
    print(f"Count: {count}")
    count = count + 1

Pattern 2: Accumulator Pattern

total = 0
number = 1
while number <= 10:
    total = total + number
    number = number + 1
print(f"Sum: {total}")  # Output: Sum: 55

Pattern 3: Flag Pattern

found = False
search_value = 5
numbers = [1, 2, 3, 4, 5, 6]
index = 0

while not found and index < len(numbers):
    if numbers[index] == search_value:
        found = True
        print(f"Found at index {index}")
    index = index + 1

Important: Updating the Condition

Critical: You must update the variable in the condition, or you'll create an infinite loop!

# This is an infinite loop - DON'T RUN THIS!
# count = 0
# while count < 5:
#     print(count)
#     # Forgot to increment count!

# Correct version:
count = 0
while count < 5:
    print(count)
    count = count + 1  # Must update!

Infinite Loops

An infinite loop runs forever because the condition never becomes False:

# This runs forever - DON'T RUN THIS!
# while True:
#     print("This never stops!")

# To stop an infinite loop, press Ctrl+C

Sometimes infinite loops are useful (like game loops), but usually they're bugs!

Breaking Out of Loops

You can use break to exit a loop early (we'll cover this in detail later):

count = 0
while True:
    print(count)
    count = count + 1
    if count >= 5:
        break  # Exit the loop

Common Mistakes

1. Forgetting to update the condition variable: This creates an infinite loop python # Wrong - infinite loop! count = 0 while count < 5: print(count) # Missing: count = count + 1

2. Wrong condition: Make sure the condition can become False python # Wrong - always True! while 1 < 2: # This is always True! print("Looping...")

3. Using = instead of ==: Assignment vs comparison ```python # Wrong - this assigns, not compares! while count = 5: # Error! print(count)

# Correct while count == 5: # Compares print(count) ```

When to Use While Loops

Use while loops when: - You don't know how many iterations you need - You're waiting for a condition to change - You're processing data until it's empty - You're validating user input - You're implementing game loops or event loops

Try It Yourself!

Create a while loop that prints numbers from 10 down to 1:

number = 10
while number >= 1:
    print(number)
    number = number - 1

Practice Tips

• Always make sure the condition can become False
• Always update variables used in the condition
• Use for loops when you know the number of iterations
• Use while loops when the number of iterations is unknown
• Be careful of infinite loops!

What's Next?

Now you understand basic while loops! Next, we'll learn about using while loops with more complex conditions.