The range Function¶

In our simple example from the last section (shown again below), we used a list of four integers to cause the iteration to happen four times. We said that we could have used any four values. In fact, we even used four colors.

import turtle            # set up alex
wn = turtle.Screen()
alex = turtle.Turtle()

for i in [0, 1, 2, 3]:   # repeat four times
    alex.forward(50)
    alex.left(90)

wn.exitonclick()

• Lists with a specific number of integers are very common
• Python gives us special built-in range objects that can deliver a sequence of values to the for loop.
• The sequence provided by range always starts with 0.
• If you ask for range(4), then you will get 4 values starting with 0.

for i in range(4):
    # Executes the body with i = 0, then 1, then 2, then 3
for x in range(10):
    # sets x to each of ... [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

So to repeat something four times, a good Python programmer would do this:

for i in range(4):
    alex.forward(50)
    alex.left(90)

• The range function is actually a very powerful function
• But what if we want [1, 2, 3, 4]? We can do this by using a two parameter version of range where the first parameter is the starting point and the second parameter is the ending point.
• The evaluation of range(1,5) produces the desired sequence.

Here are a two examples for you to run.

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.

• Finally, suppose we want to have a sequence of even numbers.
• How would we do that? Easy, we add another parameter, a step, that tells range what to count by.
• If we wanted the first 10 even numbers we would use range(0,19,2).

Try it in codelens.