OpenEuphoria

Re: Last Element Notation

• Posted by Pete Lomax <petelomax at blueyonder.co.uk> Sep 19, 2003
• 506 views

On Fri, 19 Sep 2003 12:30:42 +0000, Al Getz <Xaxo at aol.com> wrote:

>Rob had said he doesnt like this idea because he's
>checking for negative sequence refs as an error and
>apparently he doesnt know how to change to NOT checking
>for negative numbers as errors.
I would trust Rob understands that all you need is:
	if idx<0 then idx+=3Dlength()+1 end if
followed by normal bounds checking.
>
>sequence s
>s=3D{1,2,3,4}
>
>[1]
>s=3Ds[-3,-1]
>
>[2]
>s=3Ds[$-4,$-2]
Actually, that should be s[$-2,$]   )

>The only problem that would be left then would be what
>to do about <snip>:
>
>s=3Ds[4-8,-1]
>
>which generates [-4,-1]     ?

Mirror or reverse indexing should not stop bounds checking.
In existing Eu, if length(s) is 4, then idx<=3D0 or idx>=3D5 error.
Permitting backward indexes (LOL!) would change this to:
idx<=3D-5 or idx=3D0 or idx>=3D5 error.
So it doubles the number of index values which don't cause an error,
but you still get a bounds check on anything clearly out of range, and
importantly, still on s[0].

>This would have to trigger an error by way of=20
>adding the lenght of the sequence to each number
>and comparing with zero to determine if the
>reference is indeed too negative.

As I said above, inverse or transpose indexes need just:

	if idx<0 then idx+=3Dlength()+1
	-- followed by normal index bounds checking

Whatever performance overhead that is, it is certain to be less than
remembering which [] the current $ applies to, imo.

Of course if people really like the $ symbol, and internally it simply
meant -1, they could use it in exactly the same way as proposed.=20
I have no problem with that.

Pete