[R] Zero Index Origin?

[Tony Plate]

As far as I can see, the problems with legacy functions kill the 
class-based idea.  As others have noted, if you define a class for which 
the indexing function "[" is zero-based, and then pass an object x of that 
class to a legacy function, anything that function does that assumes 1 is 
the first index and length(x) is the last index and uses "[" will 
fail.  Other problems also arise, e.g., if x is a zero-based array, and y 
is a standard array, then what are the index bases of x+y and y+x?  You 
could make this depend on the order of arguments, but I suspect this would 
lead to horrible programming situations.

Anyway, it seems that what you want to do is change the behavior of the 
indexing operation, not the behavior of the objects themselves.  So, as 
Richard A. O'Keefe suggested, a more feasible approach would be to define a 
new indexing function that was zero-based.  E.g., something like 
zsub().  So, zsub(x, 0, 0) would return the upper left element of the 
matrix x.  This would not be too hard to write (and make it work for arrays 
with any number of dimensions).  If you wanted, you could also define an 
operator to access this function, e.g., so that x^[0,0] returned the upper 
left element of the matrix x, i.e., "^[" does zero-based 
indexing.  However, this is a lot more work and requires modifying the 
source code of R.  As noted by others, this zero-based indexing operator 
would not be able to use negative indices, but you could still use the 
standard indexing operators to get that feature.

I've cursed the one-based indexing in S-derived languages on the rare 
occasions when I've wanted to do index arithmetic (i.e., work out the 
linear locations of array elements based on their indexes) -- it was a pain 
to always have to subtract one from indices before multiplying and then add 
one back in at the end.  However, this really was a minor inconvenience, 
and if I wanted I could have written a function to do my index computations 
for me.  What other more significant computations are so much easier with 
zero-based indexing?

-- Tony Plate

At Wednesday 10:43 PM 3/31/2004, Bob Cain wrote:


>Gabor Grothendieck wrote:
>
>[snip good stuff]
>
>>Of course the above is motherhood and some specific examples
>>might put a sharper edge to the discussion.
>
>I really appreciate your point of view on this and think you
>are probably right.  A question I have from my very limited
>understanding yet of OO is whether such objects could be
>passed to legacy functions with any expectation of correct
>results.
>
>
>Thanks,
>
>Bob
>--
>
>"Things should be described as simply as possible, but no
>simpler."
>
>                                              A. Einstein
>
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