3.7.1. Looping using while

The while instruction repeats a code block as long as a condition is true. Its syntax is:

while condition:
    instruction1()
    instruction2()
    instruction3()
    ...

where condition is a boolean variable. Each repetition of a code block is called an iteration.

Here is an example with five iterations of a code block:

i = 0

while i < 5:
    print(f"Now, the variable i is {i}.")
    i += 1

print("done")

Now, the variable i is 0.
Now, the variable i is 1.
Now, the variable i is 2.
Now, the variable i is 3.
Now, the variable i is 4.
done

We see that as long as i was strictly lower than 5, the while instruction executed the code block. When i equaled 5, the (i < 5) condition evaluated to False, and therefore the while instruction stopped executing the code block.

Here is a practical example where we took some measurements in metres and stored them in a list. We want to convert this list to another list where the measurements are in millimetres instead:

# Measurements in meters:
meters = [0.329, 0.009, 0.210, 0.726, 0.686, 0.912, 0.285, 0.833, 0.334, 0.165]

# Create an empty list of the same measurements in millimeters, which we will
# fill up using a while loop.
millimeters = []

# Multiply each element of meters by 1000, and append it to the millimeters list.
i = 0
while i < len(meters):
    millimeters.append(meters[i] * 1000)
    i += 1

# Done.
millimeters

[329.0, 9.0, 210.0, 726.0, 686.0, 912.0, 285.0, 833.0, 334.0, 165.0]

Good practice: Looping

While this example works perfectly well and is indeed a correct demonstration of how while works, we will see later that for this specific example, other methods such as using for or NumPy would be less error-prone and faster.