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4.7. 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()
It turns out that generating lists with a specific number of integers is a very common thing to do, especially when you
want to write simple
for loop controlled iteration. Even though you can use any four items, or any four integers for that matter, the conventional thing to do is to use a list of integers starting with 0.
In fact, these lists are so popular that Python gives us special built-in
that can deliver a sequence of values to
for loop. When called with one parameter, the sequence provided by
range always starts with 0. If you ask for
range(4), then you will get 4 values starting with 0. In other words, 0, 1, 2, and finally 3. Notice that 4 is not included since we started with 0. Likewise,
range(10) provides 10 values, 0 through 9.
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]
Computer scientists like to count from 0!
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
when it comes to
creating sequences of integers. It can take one, two, or three parameters. We have seen
the simplest case of one parameter such as
range(4) which creates
[0, 1, 2, 3].
But what if we really want to have the sequence
[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. What happened to the 5?
In this case we interpret the parameters of the range function to mean
range(start,beyondLast), where beyondLast means an index past the last index we want. In the 2-parameter version
of range, that is the last index included + 1.
Why in the world would range not just work like range(start,
stop)? Think about it like this. Because computer scientists like to
start counting at 0 instead of 1,
range(N) produces a sequence of
things that is N long, but the consequence of this is that the final
number of the sequence is N-1. In the case of start,
stop it helps to simply think that the sequence begins with start and
continues as long as the number is less than stop.
The range function is lazy: It produces the next element only when needed.
With a regular Python 3 interpreter, printing a range does not calculate all the elements.
To immediately calculate all the elements in a range,
wrap the range in a list, like
Activecode is not designed to work on very long sequences, and it may allow you to be
sloppy, avoiding the list function, and see the elements in the range with
Here are two examples for you to run. Try them and then add another line below to create a sequence starting at 10 and going up to 20 (including 20).
Codelens will help us to further understand the way range works. In this case, the variable
i will take on values
produced by the
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. 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(0,19,2). The most general form of the range is
range(start, beyondLast, step). You can also create a sequence of numbers that
starts big and gets smaller by using a negative value for the step parameter.
Try it in codelens. Do you see why the first two statements produce the same result?
Check your understanding