[R] Retaining attributes of columns of a data frame when subsetting.

[Rui Barradas]

Hello,

Sorry, you're right, in the method it's x, X is the test dataframe.
Repost:

`[.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
}


The (frequent) error comes from tests where a X was created in the 
globalenv and found by the method.

Rui Barradas

Às 22:55 de 20/10/19, Rolf Turner escreveu:
> 
> On 21/10/19 1:15 AM, Rui Barradas wrote:
> 
>> Hello,
>>
>> Richard's idea is good but shouldn't it be `[.myclass` instead?
> 
> Yes, I kind of thought that, and cobbled together something on that 
> basis that seemed to work.  However my code was rather a hodge-podge.  I 
> kept having to work around errors that I didn't understand.
> 
> You seem to have a much clearer understanding of what's going on, and 
> your code is much cleaner than what I came up with.
> 
>>
>>
>> `[.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
>> }
> 
> But in the foregoing there seems to me to be some inconsistency in the 
> use of "X" and "x".
> 
> Should not the first function argument be "X" rather than "x"?  Or 
> perhaps the "X" symbols in the code should be replaced by "x"?  As in:
> 
>      SaveAt <- lapply(x, attributes)
>      x <- NextMethod()
>      ....
>      ....
> 
> Or am I misunderstanding what's going on (as is so often
> the case! :-( )?
> 
>> X <- data.frame(a = letters[1:5], x = 1:5)
>> class(X) <- c("myclass", class(X))
>> attr(X$a, "attr_a1") <- "first_a"
>> attr(X$a, "attr_a2") <- "second_a"
>> str(X)
>>
>> ok <- c(1, 3, 4)
>> X <- X[ok, ]
>> str(X)
>>
>>
>> Hope this helps,
> 
> Quite a lot!  But I'd appreciate clarification w.r.t. the misgiving that 
> I expressed above.
> 
> Thanks.
> 
> cheers,
> 
> Rolf
>