Exercise: Looping using for

3.7.2.1. Exercise: Looping using for#

In section Looping using while, we took some measurements in metres, that we needed to convert to millimeters.

meters = [0.329, 0.009, 0.210, 0.726, 0.686, 0.912, 0.285, 0.833, 0.334, 0.165]

Repeat this example, but using a for loop instead of a while loop.

Hide code cell content 
# Create an empty list of the same measurements in millimeters, that we will
# fill up using a for loop.
millimeters = []

# Multiply each element of meters by 1000, and append it to the millimeters list.
for one_measurement in meters:
    millimeters.append(one_measurement * 1000)

# Done.
millimeters

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

Good practice: for vs while

Note how the solution of this exercise is much clearer using for than while: we don’t need to initialize an index at 0 before starting the loop, and we don’t need to manually increment the index at the end of each loop. Therefore, in this case, using a for loop is less error-prone than using a while loop.

Generally:

  • We use a for loop when we know the number of iterations in advance (in the case above, this is the length of the list);

  • We use a while loop when we don’t know in advance when the loop will stop (e.g., if the stop condition is the result of a calculation).