[R] The end of Matlab

hadley wickham h.wickham at gmail.com
Fri Dec 12 18:23:34 CET 2008

On Fri, Dec 12, 2008 at 11:18 AM, Duncan Murdoch <murdoch at stats.uwo.ca> wrote:
> On 12/12/2008 11:38 AM, hadley wickham wrote:
>> On Fri, Dec 12, 2008 at 8:41 AM, Duncan Murdoch <murdoch at stats.uwo.ca>
>> wrote:
>>> 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?
>> Ooops, yes.  I meant it wouldn't require indexing to use unevaluated
>> expression.
>>>> 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.
>> That's true, although I suspect many contributed [.methods eventually
>> delegate to base methods and might work without further modification.
>>> (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?)
>> Yes, that's exactly what I was thinking, although you'd have to put
>> some thought into the conventions - would it be better to pass in the
>> length of the vector instead of a vector of indices?  Should all
>> iterators return logical vectors?  That way you could do x[evens() &
>> last(5)] to get the even indices out of the last 5, as opposed to
>> x[evens()][last(5)] which would return the last 5 even indices.
> Actually, I don't think so.  "evens() & last(5)" would fail to evaluate,
> because you're trying to do a logical combination of two functions, not of
> two logical vectors.  Or are we going to extend the logical operators to
> work on iterators/selectors too?

Oh yes, that's a good point.  But wouldn't the following do the job?

"&.selector" <- function(a, b) {
  function(n) a(n) & b(n)


"&.selector" <- function(a, b) {
  function(n) intersect(a(n), b(n))

depending on whether selectors return logical or numeric vectors.
Writing functions for | and ! would be similarly easy.  Or am I
missing something?



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