[Rd] Drop single-dimensional array

Peter Dalgaard pdalgd at gmail.com
Wed Sep 8 22:51:40 CEST 2010

On 09/08/2010 08:54 PM, Arni Magnusson wrote:
> 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. 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.

Well, you're missing c(x), but you may still have a point...

1d arrays was something of an afterthought, introduced for the ability
to have named dimnames. Had this been thought of when S was designed,
then maybe vectors and 1d arrays had been the same, but when it
happened, it was not an option to modify the basic vector structures.

drop() is documented to drop extents of length one, of which there are
none in the 1d-array case, and so it is taken as a no-op. A slightly
different design would be to drop in a strict array sense, i.e.
dim=c(1,1,12) would drop to dim=12, not to a vector of length 12, and
THEN if the result is 1d, convert to vector (this could even be made
optional, with some reason).

I don't think it's at the top of anyone's agenda, though. Meanwhile,
c(x) should get you there soon enough.


> 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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Peter Dalgaard
Center for Statistics, Copenhagen Business School
Phone: (+45)38153501
Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com

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