On Sun, 3 Nov 2019 at 22:12, Rolf Turner <r.tur...@auckland.ac.nz> 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?
The thing is...
test <- function(x = y * 2) {
y <- 1
x
}
test()
# 2
Lazy evaluation magic.
Iñaki
______________________________________________
R-devel@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-devel
