Loops are one of the fundamental constructs that enable us to control the flow of our program. A while loop is a type of loop that repeats a block of code while a specific condition is true. While loops are perfect for when we want to repeat code an unknown number of times.

We actually use while loops in our everyday lives all the time:

• While the road is straight, drive forward.
• While the temperature is at or below 68 degrees, keep the heat on.

In these examples, we’re not quite sure when the road will curve or when exactly the temperature is going to change. So, we continue driving straight until the road curves, or we keep the heat on until the temperature is higher than 68 degrees.

A while loop in Python looks like this:

while condition:
    // code to execute
    // while the condition is true

If the condition evaluates to true, the code inside the loop will execute. Once the condition evaluates to false, the computer will exit the while loop and continue on with the program. Click through the slides to walk through a while loop in action.

Look at the example program below. In plain English, it says while there are more than 0 tacos, eat a taco. Run the program and explore with these guiding questions:

• What happens if you change the initial value of num_tacos?
• What happens if you change the condition of the while loop to num_tacos < 0?
• When does the final print statement run? Why does it run at that moment?
• What happens if you change the while loop to an if statement?

Take a look at the While Loops Guessing Game below. Before running the program, predict what you think will happen. What is the while loop condition in plain English? When will the while loop end? (If you are unfamiliar with the != symbol, check out the Booleans and Logical Operators tutorial.

Infinite Loops

When writing a while loop, beware of the infinite loop! An infinite loop occurs when the condition is always true. When there is an infinite loop, the computer will attempt to run the while loop forever, which will ultimately result in crashing the browser or an error.

Let’s revise our tacos program slightly. Look closely - can you figure out why this program would result in an infinite loop?

num_tacos = 5

while num_tacos > 0:
    print("There are " + str(num_tacos) + " left.")

A hint that you might have an infinite loop on your hands is if you don’t change the value of the variable in the condition. In the above program, we never change the value of num_tacos in our loop. This means that num_tacos will always be 5, which also means that the condition num_tacos > 0 will always be true. When the computer runs the program, it will end up repeating the loop an infinite number of times because the condition always evaluates to true and there is no way to exit the loop.

Your Turn!

Practice your hand at while loops by completing any (or all!) of the following challenges:

Level 1: Write a while loop that prints a countdown from 10.

Level 2: Write a program that goes from 100 to 0 at different intervals specified by the user. You can use the int(input("prompt")) command to get a number from the user. Your sample output might look like this:

Decrease by how much? 50
Number is now 50
Decrease by how much? 25
Number is now 25
Decrease by how much? 2
Number is now 23
Decrease by how much? 23
Number is now 0