In our first example of a while loop, we counted down from 10 to 0. If we were to consider doing this with a for loop, we would need to construct our own series of numbers to loop through them.
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 range objects that can deliver a sequence of values to the 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.
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, convert the range object to a list, like list(range(4)). 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 print(range(4)).
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.
This command generates the sequence with just the number 2 because the first parameter (2) tells range where to start, the second number tells range where to end (before 5) and the third number tells range how many numbers to skip between elements (8). Since 10 >= 5, there is only one number in this sequence.
Yes: If we take steps of -5, not worrying about the ending, we get 20, 15, 10, 5, 0, .... The limit 3 is past the 5, so the range sequence stops with the 5.
The sequence produced has steps of 4: 2, 6, 10. The next would be 14, but it is not before the limit 12. There are other limit choices past 10, but not past 14.