From murdoch@dunc@n @end|ng |rom gm@||@com  Mon Nov  4 01:06:46 2019
From: murdoch@dunc@n @end|ng |rom gm@||@com (Duncan Murdoch)
Date: Sun, 3 Nov 2019 19:06:46 -0500
Subject: [Rd] Puzzled about a new method for "[".
In-Reply-To: <8faee382-b436-ce7d-19b9-b6bdb65f2a66@auckland.ac.nz>
References: <74a6cf10-3266-fef8-9ea0-0ba6d36db807@auckland.ac.nz>
 <b795d8d3-d20b-0576-9d8d-011307e51707@gmail.com>
 <8faee382-b436-ce7d-19b9-b6bdb65f2a66@auckland.ac.nz>
Message-ID: <ca5c37d7-1951-d35f-1d20-5a74b8976969@gmail.com>

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 "..." .

Duncan Murdoch

> 
> ???
> 
> As I said, this *seems* to work OK, by I cannot work through what the
> implications might be.
> 
> Can anyone reassure me?
> 
> cheers,
> 
> Rolf
>



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