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
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