[R] data.table evaluating columns

[Tom Short]

On Tue, Mar 2, 2010 at 7:09 PM, Rob Forler <rforler at uchicago.edu> wrote:
> Hi everyone,
>
> I have the following code that works in data frames taht I would like tow
> ork in data.tables . However, I'm not really sure how to go about it.
>
> I basically have the following
>
> names = c("data1", "data2")
> frame = data.frame(list(key1=as.integer(c(1,2,3,4,5,6)),
> key2=as.integer(c(1,2,3,2,5,6)),data1 =  c(3,3,2,3,5,2), data2=
> c(3,3,2,3,5,2)))
>
> for(i in 1:length(names)){
> frame[, paste(names[i], "flag")] = frame[,names[i]] < 3
>
> }
>
> Now I try with data.table code:
> names = c("data1", "data2")
> frame = data.table(list(key1=as.integer(c(1,2,3,4,5,6)),
> key2=as.integer(c(1,2,3,2,5,6)),data1 =  c(3,3,2,3,5,2), data2=
> c(3,3,2,3,5,2)))
>
> for(i in 1:length(names)){
> frame[, paste(names[i], "flag"), with=F] = as.matrix(frame[,names[i],
> with=F] )< 3
>
> }

Rob, this type of question is better for the package maintainer(s)
directly rather than R-help. That said, one answer is to use list
addressing:

for(i in 1:length(names)){
    frame[[paste(names[i], "flag")]] = frame[[names[i]]] < 3
}

Another option is to manipulate frame as a data frame and convert to
data.table when you need that functionality (conversion is quick).

In the data table version, frame[,names[i], with=F] is the same as
frame[,names[i], drop=FALSE] (the answer is a list, not a vector).
Normally, it's easier to use [[]] or $ indexing to get this. Also,
fname[i,j] <- something assignment is still a bit buggy for
data.tables.

- Tom

Tom Short