How to write an R function that evaluates an expression within a data-frame

Question

Puzzle for the R cognoscenti: Say we have a data-frame:

df <- data.frame( a = 1:5, b = 1:5 )

I know we can do things like

Answer 1

A late entry, but the data.table approach and syntax would appear to be what you are after. This is exactly how [.data.table works with the j, i and by arguments.

If you need it in the form fn(x,expr), then you can use the following

library(data.table)

DT <- data.table(a = 1:5, b = 2:6)

`[`(x=DT, j=a)

## [1] 1 2 3 4 5

 `[`(x=DT, j=a * b)
## [1]  2  6 12 20 30

I think it is easier to use in more native form

DT[,a]
## [1] 1 2 3 4 5

and so on. In the background this is using substitute and eval