While Loops

Breaking the linear path

Up until now, your code runs like a simple waterfall: it starts at line 1, executes line 2, line 3, and then finishes.

But what if you need to repeat a task? If you need to print "Hello" ten thousand times, you do not want to write 10,000 print statements.

You need a loop.

The while loop

A while loop runs a block of code over and over again as long as a condition remains True.

python
countdown = 5

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

print("Blastoff!")

Here is exactly how Python reads this:

1. Is countdown > 0? Yes (it's 5). Print, and subtract 1.
2. Is countdown > 0? Yes (it's 4). Print, and subtract 1.
3. ...
4. Is countdown > 0? Yes (it's 1). Print, and subtract 1.
5. Is countdown > 0? False (it's 0).

The moment the condition becomes False, the loop breaks and Python drops down to print "Blastoff!".

The danger of the Infinite Loop

If you forget to update the variable inside the loop (like forgetting countdown = countdown - 1), the condition will never become False.

python
countdown = 5

while countdown > 0:
    print("This will run forever!")
    # Missing subtraction here!

This creates an Infinite Loop. Your program will just print that line millions of times until you physically force-quit it or your computer runs out of memory.

Always make sure your loop has a clear, guaranteed way to finish!

What this lesson should give you

After this lesson, you should understand how to:

• write a while loop that repeats an action
• establish a condition that controls when the loop stops
• update variables inside the loop to avoid an infinite loop
• understand the order of execution as Python flows backward up to test the condition again