On 12/28/2010 9:43 AM, Tim Hesterberg wrote:
Having aperm() return an object of the same class is dangerous, there
are undoubtedly classes for which that is not appropriate, producing an
illegal object for that class or quietly giving incorrect results.
OK. I can see that my initial proposal would be dangerous for xtabs
objects without further
modifications and that it is unwise to change default behavior in base
functions without
very strong reasons.
Three alternatives are to:
* add the keep.class option but with default FALSE
This first option is the minimally invasive corrective surgery.
This would put the burden on the user (or package writer), but at least
make it known
that keep.class=TRUE is an option. This version is
## add keep.class, non-generic
aperm <- function (a, perm, resize = TRUE, keep.class=FALSE)
{
if (missing(perm))
perm <- integer(0L)
result <- .Internal(aperm(a, perm, resize))
if(keep.class) class(result) <- class(a)
result
}
* make aperm a generic function
- without a keep.class argument
- with a ... argument
- methods for classes like table could have keep.class = TRUE
This would be much better, as long as an aperm.table method was added to
base, to complement table() itself,
and gives the desired behavior for table objects by default.
This version seems to be:
## make generic, with ...
aperm <- function(a, ...)
UseMethod("aperm", ...)
aperm.default <- function (a, perm, resize = TRUE, ...)
{
if (missing(perm))
perm <- integer(0L)
.Internal(aperm(a, perm, resize))
}
aperm.table <- function(a, perm, resize=TRUE, keep.class=TRUE, ...)
{
result <- aperm.default(a, perm, resize=resize)
if(keep.class) class(result) <- class(a)
result
}
But it throws an error, maybe because I haven't redefined aperm as a
generic:
> UCB <- aperm(UCBAdmissions, c(2,1,3))
Error in aperm(UCBAdmissions, c(2, 1, 3)) :
'...' used in an incorrect context
The .table method does work as desired:
> UCB <- aperm.table(UCBAdmissions, c(2,1,3))
> str(UCB)
table [1:2, 1:2, 1:6] 512 89 313 19 353 17 207 8 120 202 ...
- attr(*, "dimnames")=List of 3
..$ Gender: chr [1:2] "Male" "Female"
..$ Admit : chr [1:2] "Admitted" "Rejected"
..$ Dept : chr [1:6] "A" "B" "C" "D" ...
>
* make aperm a generic function
- without a keep.class argument
- with a ... argument
- default method have keep.class = TRUE
The third option would give the proposed behavior by default, but
allow a way out for classes where the behavior is wrong. This puts
the burden on a class author to realize the potential problem with
aperm, so my preference is one of the first two options.
aperm() was designed for multidimensional arrays, but is also useful for
table objects, particularly
with the lattice, vcd and vcdExtra packages. But aperm() was designed
and implemented before other
related object classes were conceived, and I propose a small tune-up to
make it more generally useful.
The problem is that aperm() always returns an object of class 'array',
which causes problems for methods
designed for table objects. It also requires some package writers to
implement both .array and .table
methods for the same functionality, usually one in terms of the other.
Some examples of unexpected, and initially perplexing results (when only
methods for one class are implemented)
are shown below.
library(vcd)
pairs(UCBAdmissions, shade=TRUE)
UCB<- aperm(UCBAdmissions, c(2, 1, 3))
# UCB is now an array, not a table
pairs(UCB, shade=TRUE)
There were 50 or more warnings (use warnings() to see the first 50)
# fix it, to get pairs.table
class(UCB)<- "table"
pairs(UCB, shade=TRUE)
Of course, I can define a new function, tperm() that does what I think
should be the expected behavior:
# aperm, for table objects
tperm<- function(a, perm, resize = TRUE) {
result<- aperm(a, per, resize)
class(result)<- class(a)
result
}
But I think it is more natural to include this functionality in aperm()
itself. Thus, I propose the following
revision of base::aperm(), at the R level:
aperm<- function (a, perm, resize = TRUE, keep.class=TRUE)
{
if (missing(perm))
perm<- integer(0L)
result<- .Internal(aperm(a, perm, resize))
if(keep.class) class(result)<- class(a)
result
}
I don't think this would break any existing code, except where someone
depended on coercion to an array.
The drop-in replacement for aperm would set keep.class=FALSE by default,
but I think TRUE is more
natural.
FWIW, here are the methods for table and array objects
>from my current (non-representative) session.
methods(class="table")
[1] as.data.frame.table barchart.table* cloud.table*
contourplot.table* dotplot.table*
[6] head.table* levelplot.table* pairs.table*
plot.table* print.table
[11] summary.table tail.table*
Non-visible functions are asterisked
methods(class="array")
[1] anyDuplicated.array as.data.frame.array as.raster.array*
barchart.array* contourplot.array* dotplot.array*
[7] duplicated.array levelplot.array* unique.array
--
Michael Friendly Email: friendly AT yorku DOT ca
Professor, Psychology Dept.
York University Voice: 416 736-5115 x66249 Fax: 416 736-5814
4700 Keele Street Web:http://www.datavis.ca
Toronto, ONT M3J 1P3 CANADA
--
Michael Friendly Email: friendly AT yorku DOT ca
Professor, Psychology Dept.
York University Voice: 416 736-5115 x66249 Fax: 416 736-5814
4700 Keele Street Web: http://www.datavis.ca
Toronto, ONT M3J 1P3 CANADA
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