4.6. The range Function¶

In our simple example from the last section (shown again below), we used a loop to draw a square.

import turtle

for i in range(0, 4, 1):
    turtle.forward(50.0)
    turtle.left(90.0)

Each iteration of the loop the value of the variable i changes. To see it change, we can declare the variable and print it out during the loop.

i: int

for i in range(0, 4, 1):

    print(str(i))

​

In the above program the variable i has the value 0, 1, 2, and 3. These values are controlled by the three inputs to the range function, 0, 4, and 1. The first input is the start value, 0 in the above example. The start is the first value that the variable i will have. The second input is the stop value, 4 in the above example. The stop determines the last value the variable i will have. Note that the stop value is exclusive. The variable i will never be the stop value. The third input is the step value, 1 in the above example. The step is how much the variable i changes by each iteration of the loop.

Codelens will help us to further understand the way range works. In this case, the variable i will take on values produced by the range function.

Suppose we want to have a sequence of even numbers. How would we do that? Easy, change the range function’s step input. For even numbers we want to start at 0 and count by 2’s. So if we wanted the first 10 even numbers we would use range(0, 20, 2).

i: int

for i in range(0, 20, 2):

    print(str(i))

​

You can also create a sequence of numbers that starts big and gets smaller by using a negative value for the step parameter.

i: int

for i in range(10, 0, -1):

    print(str(i))

​

Check your understanding