[Edu-sig] Confusion with slice notation

[Trent A. Oliphant]

Here is where I get caught each time with list notation.

(I realized as I was writing this it isn't the slice notation but the index
notation that gets me)


x[:3] returns a list with the first three elements in it.
x[-3:] returns a list with the last three elements in it.

x[2] returns the third element in a list, but
x[-3] returns the third to the last element in a list

To illustrate it another way

x = [1,2,3,4,5,6,7,8,9]
y = [9,8,7,6,5,4,3,2,1]

The way I look at it (and this may be where the problem lies) is that the
[-n] notation is an inverse of sorts of the [n] notation.

Slicing does work that way:
x[:3] = [1,2,3] (3 elements)
y[-3:] = [3,2,1] (3 elements)

However, indexing doesn't
x[3] = 4  (0 based indexing)
y[-3] = 3 (could be viewed as -1 based indexing)

So, I can't take my model for the slices and translate it to indexing, even
though the notation is very similar.

It just seems a bit inconsistent to me, and as I said earlier, I haven't
been able to get it.  I am not very good at just memorizing something and
saying that's just the way it is, I need more, and I haven't figured out
what that more is for this issue.

Just some food for thought.

Trent Oliphant