While Loop

Introduction

Loops allow programs to repeat a block of statements multiple times. The while loop is an example of an **indefinite loop** or condition-controlled loop. It repeatedly runs its block of code as long as its condition expression evaluates to True.

Syntax of While Loop

The syntax of a while loop in Python is:

while condition:
    # Indented block of statements
    statements

The condition is evaluated first. If it is True, the loop body runs. After executing the body, the condition is evaluated again. This repeats until the condition becomes False.

Three Key Elements of a Loop

To write a working loop that terminates correctly, we need three key elements:

1. Initialization: Setting up a starting value (e.g., count = 1).
2. Condition Check: The test expression used by the loop (e.g., while count <= 5).
3. Update (Increment/Decrement): Modifying the state so the loop makes progress towards termination (e.g., count += 1).

Infinite Loops and How to Avoid Them

If the loop condition is always True and never updated to False, the loop will run forever, creating an infinite loop.

# Infinite Loop Example (Avoid this!)
x = 1
while x <= 5:
    print(x)  # Runs forever because x is never incremented!

Code Examples

Example 1: Printing numbers 1 to 5

count = 1
while count <= 5:
    print(count)
    count += 1
print("Done!")

Example 2: Adding user-entered numbers until 0 is typed

total = 0
val = int(input("Enter a number (0 to exit): "))

while val != 0:
    total += val
    val = int(input("Enter a number (0 to exit): "))

print("Total sum:", total)

Summary

A while loop executes as long as its condition evaluates to True. Correct initialization, clear condition checks, and updating variables inside the loop body are required to prevent programs from entering infinite loops.