[R] The end of Matlab

[Duncan Murdoch]

On 12/12/2008 8:25 AM, hadley wickham wrote:
>> From which you might conclude that I don't like the design of subset, and
>> you'd be right.  However, I don't think this is a counterexample to my
>> general rule.  In the subset function, the select argument is treated as an
>> unevaluated expression, and then there are rules about what to do with it.
>>  (I.e. try to look up name `a` in the data frame, if that fails, ...)
>>
>> For the requested behaviour to similarly fall within the general rule, we'd
>> have to treat all indices to all kinds of things (vectors, matrices,
>> dataframes, etc.) as unevaluated expressions, with special handling for the
>> particular symbol `end`.
> 
> Except you wouldn't have to necessarily change indexing - you could
> change seq instead.  Then 5:end could produce some kind of special
> data structure (maybe an iterator) that was recognised by the various
> indexing functions. 

Ummm, doesn't that require changes to *both* indexing and seq?

> This would still be a lot of work for not a lot
> of payoff, but it would be a logically consistent way of adding this
> behaviour to indexing, and the basic work would make it possible to
> develop other sorts of indexing, eg df[evens(), ], or df[last(5),
> last(3)].

I agree:  it would be a nice addition, but a fair bit of work.  I think 
it would be quite doable for the indexable things in the base packages, 
but there are a lot of contributed packages that define [ methods, and 
those methods would all need to be modified too.

(Just to be clear, when I say doable, I'm thinking that your iterators 
return functions that compute subsets of index ranges.  For example, 
evens() might be implemented as

evens <- function() {
   result <- function(indices) {
     indices[indices %% 2 == 0]
   }
   class(result) <- "iterator"
   return(result)
}

and then `[` in v[evens()] would recognize that it had been passed an 
iterator, and would pass 1:length(v) to the iterator to get the subset 
of even indices.  Is that what you had in mind?)

Duncan