[Rd] Drop single-dimensional array

[William Dunlap]

> -----Original Message-----
> From: r-devel-bounces at r-project.org 
> [mailto:r-devel-bounces at r-project.org] On Behalf Of Arni Magnusson
> Sent: Wednesday, September 08, 2010 11:55 AM
> To: r-devel at r-project.org
> Cc: Simon Urbanek
> Subject: [Rd] Drop single-dimensional array
> 
> Hi Simon, thank you for the concise reply.
> 
> Do you mean the reported behavior of drop() is not a bug?
> 
> It looks like a borderline bug to me (see below), but I'm not 
> the judge of 
> that. If this is the intended behavior and serves an actual 
> purpose, then 
> that could be explicitly documented in a \note{} on the help page.
> 
> Such a note would slightly reduce the surprise of users 
> running into this 
> behavior.
> 
> This is related to the oddity that one-dimensional arrays are:
> 
>    array(month.abb, dim=c(1,1,12))  # array
>    array(month.abb, dim=c(1,12))    # matrix
>    array(month.abb, dim=12)         # array
> 
> Firstly, one would expect the pattern to be 
> array-matrix-vector. 

I would expect the array() constructor function to return
something of class array.  (I guess matrix is acceptable
because it inherits from array).

One dimensional arrays and vectors act very similarly
when used in R code.  E.g., names(array1d) gives you
the same thing as dimnames(array1d)[[1]].  Accessing
them from C code probably shows more differences.
Are there features beyond drop's behavior that are
causing you to be concerned about differences between
the 2 types?

drop(x) and x[..., drop=TRUE] have always bugged me
because you couldn't control which dimensions got
dropped.  E.g., if you have a n by 2 by 3 array
and want to get the 3rd n by 2 slice of it as a matrix
you can do
   array[,,3,drop=TRUE]
except when n==1.  It would be nice to be able to say
"drop the 3rd dimension, throwing an error if that
dimension is not 1".

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com 

> Secondly, 
> it's easy to drop() the three-dimensional and two-dimensional 
> objects, but 
> drop() does nothing to the one-dimensional array. Instead, it 
> takes an 
> unintuitive combination of methods to convert a 
> single-dimensional to a 
> vector, while retaining its names. Or I may well be missing something 
> obvious.
> 
> Best regards,
> 
> Arni
> 
> 
> 
> On Wed, 8 Sep 2010, Simon Urbanek wrote:
> 
> > wrong address - did you mean R-devel?
> > Simon
> >
> >
> >
> > On Sep 6, 2010, at 8:35 AM, Arni Magnusson wrote:
> >
> >> Bug or not, I was surprised by this behavior:
> >>
> >>  x <- tapply(chickwts$weight, chickwts$feed, median)
> >>  x  # array ... I'd like to convert to vector with named elements
> >>  drop(x)             # what, still an array?
> >>  drop(as.matrix(x))  # this works
> >>  drop(t(x))          # this works
> >>
> >> I was expecting drop(x) to return a vector, and I suspect 
> many R users 
> >> would too. The title in help(drop), "Drop Redundant Extent 
> >> Information", suggests that such a simple array would be 
> converted to a 
> >> vector.
> >>
> >> Reading through the help page, I note that this is perhaps 
> not a clear 
> >> bug, but somewhat unclear behavior.
> >>
> >> The most compact way to break the vector out of its 
> eggshell seems to 
> >> be
> >>
> >>  t(x)[,]
> >>
> >> but drop(x) would be much easier to read and write. 
> There's nothing 
> >> particularly matrix about x, so it's not obvious that the 
> conversion 
> >> should involve as.matrix(x).
> >>
> >> Thanks,
> >>
> >> Arni
> >
> 
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