[Rd] Format printing inside a matrix

[Pages, Herve]

On 7/7/19 17:41, Jialin Ma wrote:
> Hi Abby,
> 
> Thanks a lot for your paraphrasing and your suggestion!
> 
> The problem of wrapping the list into a S3/S4 object, i.e. subclassing array
> or matrix, is that one also has to define a bunch of methods for subsetting,
> joining, etc, in order to make it behave like a list array. The reason is that
> most R functions for subsetting, joining, etc. do not preserve class
> attributes of the input, which is possibly by design.

Is it? One could argue that a more sensible behavior would be that
things like `[`(..., drop=FALSE), rbind(), cbind(), etc... preserve
the class attribute.

Interestingly t() does that:

   m <- matrix(1:6, nrow=2)
   class(m) <- "foo"

   m
   #      [,1] [,2] [,3]
   # [1,]    1    3    5
   # [2,]    2    4    6
   # attr(,"class")
   # [1] "foo"

   t(m)
   #      [,1] [,2]
   # [1,]    1    2
   # [2,]    3    4
   # [3,]    5    6
   # attr(,"class")
   # [1] "foo"

but not aperm() (which in this case would be expected to be
equivalent to t()):

   aperm(m, 2:1)
   #      [,1] [,2]
   # [1,]    1    2
   # [2,]    3    4
   # [3,]    5    6

Reshaping also preserves the class:

   dim(m) <- c(6, 1)
   m
   #      [,1]
   # [1,]    1
   # [2,]    2
   # [3,]    3
   # [4,]    4
   # [5,]    5
   # [6,]    6
   # attr(,"class")
   # [1] "foo"

So if it makes sense for t() and reshaping, it's not clear why it
wouldn't for [, aperm(), cbind(), etc...?

H.


> It is not desirable if a
> simple matrix subsetting will remove the class attributes of the object.
> 
> There are also many other common functions that are meant to drop the
> attributes of the input, e.g. lapply, apply -- they are not generics and it is
> not wise to override them.
> 
> Overall, introducing a new S3/S4 class often brings some extra price in order
> to maintain the correct behavior, which is why I tend to avoid them unless it
> is necessary.
> 
> Best regards,
> Jialin
> 
> On Sunday, July 7, 2019 4:43:59 PM PDT Abby Spurdle wrote:
>> contains an array of question marks, which
>> isn't helpful.
>>
>> Would it be possible for the print method for both matrices and arrays
>> (conditional on having a list type), call the format method for each
>> object, possibly creating a character matrix?
>> Presumably there are other approaches, but the main thing is that the
>> output is useful and it's easy for R users to control the way objects (in
>> matrices and arrays) are printed.
>>
>>> Or is there any other place that I can override without introducing a new
>>
>> S3 class?
>>
>> In theory, the simplest approach is to redefine the print method for
>> matrices.
>> However, that would be unacceptable in a CRAN package, probably...
>>
>> So, unless R Core change the print method, you may have to create a matrix
>> subclass.
> 
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