Question

Suppose I want to set the class on all the items in a data.table column. I feel like there should be a way to do this sort of thing from inside the DT[,j] part, but it doesn’t seem to work. I have to do it using $. Does anyone know why?

> DT <- data.table(L = letters, N = 1:26)
> sapply(DT, class)
          L           N 
"character"   "integer" 
> DT[, {class(N) <- "MyClass"}]   # Doesn't work
[1] "MyClass"
> sapply(DT, class)
          L           N 
"character"   "integer" 
> DT[, class(N) <- "MyClass"]    # Doesn't work
[1] "MyClass"
> sapply(DT, class)
          L           N 
"character"   "integer" 
> class(DT$N) <- "MyClass"    # Works
> sapply(DT, class)
          L           N 
"character"   "MyClass" 

Answer 1

DT[,N:=as.myclass(N)]

Should work