In R data.table, how do I pass variable parameters to an expression?

data.table, parameter-passing, r

Solution

An alternative to flodel's answer in the comments could be

e <- parse(text = paste0("sum(", v1, ", na.rm = TRUE)"))

b <- parse(text = v2)

rDT2 <- dt[, eval(e), by = eval(b)]

#               b    V1
# [1,]     setosa 250.3
# [2,] versicolor 296.8
# [3,]  virginica 329.4

EDIT:

And to put this into a function,

getResult <- function(dt, expr, gby){
  return(dt[, eval(expr), by = eval(gby)])
}

(dtR <- getResult(dt = dt, expr = e, gby = b))
# gives the same result as above

EDIT from Matthew: There's a subtle reason why the `paste0` and `eval` \ `quote` methods can be faster than `get` in some cases, too. One of the reasons grouping can be fast is that `data.table` inspects `j` to see which columns it uses, then only subsets those used columns (FAQ 1.12 and 3.1). It uses `base::all.vars(j)` to do that. When using `get()` in `j` the column being used is hidden from `all.vars` and `data.table` falls back to subsetting all the columns just in case the `j` expression needs them (much like when the `.SD` symbol is used in `j`, for which `.SDcols` was added to solve). If all the columns are used anyway then it doesn't make a difference, but if `DT` is say 1e7x100 then a grouped `j=sum(V1)` should be much faster than a grouped `j=sum(get("V1"))` for that reason. At least, that's what's supposed to happen, and if it doesn't then it may be a bug. If on the other hand many queries are being constructed dynamically and repeated then the time to `paste0` and `parse` might come into it. All depends really. Setting `verbose=TRUE` should print out a message about which columns have been detected as used by `j`, so that can be checked.

Problem

I am stuck with a small R issue with `data.table`. Your help is much appreciated. How do I do this: ``` getResult <- function(dt, expr, gby) { e <- substitute(expr) b <- substitute(gby) return(dt[,eval(e),by=b]) } v1 <- "Sepal.Length" v2 <- "Species" dt <- data.table(iris) rDT <- getResult(dt, sum(v1, na.rm=TRUE), v2) ``` I get following error: Error in sum(v1, na.rm = TRUE) : invalid 'type' (character) of argument Now, both `v1` and `v2` get passed from other program as character variable so I can't do this `v1<- quote(Sepal.Length)` which seems to work.

Original source