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

[William Dunlap]

> the perils certainly are not immediately apparent to me.

Here is a concrete example of a peril
 `[.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
   }

 x <- data.frame(Mat=I(matrix(101:106,ncol=2)), Vec=201:203)
 xmc <- structure(x, class=c("myclass", class(x)))
 xmc[1:2,]
Error in attributes(x[[nm]]) <- Sat[[nm]] :
  dims [product 6] do not match the length of object [4]
 x[1:2,]
  Mat.1 Mat.2 Vec
1   101   104 201
2   102   105 202

I would be surprised if extracting a column from some rows of a data.frame
gave a different result than extracting some rows from a column of a
data.frame.  The row-selecting method used by [.data.frame depends on the
class of the column.

Bill Dunlap
TIBCO Software
wdunlap tibco.com


On Mon, Nov 4, 2019 at 12:28 PM Rolf Turner <r.turner using auckland.ac.nz> wrote:

>
> On 5/11/19 3:41 AM, Hadley Wickham wrote:
>
> > For what it's worth, I don't think this strategy can work in general,
> > because a class might have attributes that depend on its data/contents
> > (e.g. https://vctrs.r-lib.org/articles/s3-vector.html#cached-sum). I
> > don't think these are particularly common in practice, but it's
> > dangerous to assume that you can restore a class simply by restoring
> > its attributes after subsetting.
>
>
> You're probably right that there are lurking perils in general, but I am
> not trying to "restore a class".  I simply want to *retain* attributes
> of columns in a data frame.
>
> * I have a data frame X
> * I attach attributes to certain of its columns;
>       attr(X$melvin,"clyde") <- 42
>    (I *don't* change the class of X$melvin.)
> * I form a subset of X:
>      Y <- X[1:100,3:10]
> * given that "melvin" is amongst columns 3 through 10 of X,
>      I want Y$melvin to retain the attribute "clyde", i.e. I
>      want attr(Y$melvin,"clyde") to return 42
>
> There is almost surely a better approach than the one that I've chosen
> (isn't there always?) but it seems to work, and the perils certainly are
> not immediately apparent to me.
>
> cheers,
>
> Rolf
>
> --
> Honorary Research Fellow
> Department of Statistics
> University of Auckland
> Phone: +64-9-373-7599 ext. 88276
>
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