[Rd] Puzzled about a new method for "[".

[Rolf Turner]

On 4/11/19 1:06 PM, Duncan Murdoch wrote:

> On 03/11/2019 6:43 p.m., Rolf Turner wrote:
>>
>> On 4/11/19 10:31 AM, Duncan Murdoch wrote:
>>
>>> On 03/11/2019 4:11 p.m., Rolf Turner wrote:
>>>>
>>>> I recently tried to write a new method for "[", to be applied to data
>>>> frames, so that the object returned would retain (all) attributes of 
>>>> the
>>>> columns, including attributes that my code had created.
>>>>
>>>> I thrashed around for quite a while, and then got some help from Rui
>>>> Barradas who showed me how to do it, in the following manner:
>>>>
>>>> `[.myclass` <- function(x, i, j, drop = if (missing(i)) TRUE else
>>>> length(cols) == 1)[{
>>>>       SaveAt <- lapply(x, attributes)
>>>>       x <- NextMethod()
>>>>       lX <- lapply(names(x),function(nm, x, Sat){
>>>>         attributes(x[[nm]]) <- Sat[[nm]]
>>>>         x[[nm]]}, x = x, Sat = SaveAt)
>>>>       names(lX) <- names(x)
>>>>       x <- as.data.frame(lX)
>>>>       x
>>>> }
>>>>
>>>> If I set class(X) <- c("myclass",class(X)) and apply "[" to X (e.g.
>>>> something like X[1:42,]) the attributes are retained as desired.
>>>>
>>>> OK.  All good.  Now we finally come to my question!  I want to put this
>>>> new method into a package that I am building.  When I build the package
>>>> and run R CMD check I get a complaint:
>>>>
>>>> ... no visible binding for global variable ‘cols’
>>>>
>>>> And indeed, there is no such variable.  At first I thought that maybe
>>>> the code should be
>>>>
>>>> `[.myclass` <- function(x, i, j, drop = if (missing(i)) TRUE else
>>>>                                          length(j) == 1)[{
>>>>
>>>> But I looked at "[.data.frame" and it has "cols" too; not "j".
>>>>
>>>> So why doesn't "[.data.frame" throw a warning when R gets built?
>>>>
>>>> Can someone please explain to me what's going on here?
>>>
>>> Defaults for parameters are evaluated in the evaluation frame of the
>>> function, at the time the parameter is first used.
>>>
>>> If you look at the source for "[.data.frame", you should see that "cols"
>>> is defined there as a local variable.  The "drop" argument shouldn't be
>>> used until it is.  (There's a call to "missing(drop)" early in the
>>> source that doesn't count:  it doesn't evaluate "drop", it just checks
>>> whether it is specified by the caller.)
>>
>>
>> OK.  As I understand what you're saying, the reason there isn't a
>> "no visible binding" problem in [.data.frame is that "cols" *is* defined
>> in the body of the function.  Whereas, in my method, "cols" does not get
>> defined anywhere in the function, and thus triggers the warning.
>>
>> I guess that a workaround would be to do a dummy assignment, like unto
>> cols <- 42 at the start of the code for my method.
>>
>> (a) Are there perils involved with this strategy?
> 
> Only that 42 might not be the right value.
> 
>>
>> (b) Is there anything wrong with my current strategy of replacing
>>
>>      drop = if (missing(i)) TRUE else length(cols) == 1)
>>
>> by
>>
>>      drop = if (missing(i)) TRUE else length(j) == 1)
> 
> [.data.frame is pretty complicated, and I haven't read it closely enough 
> to know if this is equivalent.  I would suggest you consider not 
> including "drop" at all, just implicitly including it in "..." .

OK.  I'll try that!

Thanks.

cheers,

Rolf

-- 
Honorary Research Fellow
Department of Statistics
University of Auckland
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