How to use data.table inside a function?

Question

As a minimal working example, for instance, I want to be able to dynamically pass expressions to a data.table object to create new columns or modify existing ones:

dt <- data.table(x = 1, y = 2)
dynamicDT <- function(...) {
    dt[, list(...)]
}
dynamicDT(z = x + y)

I was expecting:

   z
1: 3

but instead, I get the error:

Error in eval(expr, envir, enclos) : object 'x' not found

So how can I fix this?

Attempts:

I've seen this post, which suggests using quote or substitute, but

> dynamicDT(z = quote(x + y))
Error in `rownames<-`(`*tmp*`, value = paste(format(rn, right = TRUE),  : 
  length of 'dimnames' [1] not equal to array extent

or

> dynamicDT <- function(...) {
+     dt[, list(substitute(...))]
+ }
> dynamicDT(z = x + y)
Error in prettyNum(.Internal(format(x, trim, digits, nsmall, width, 3L,  : 
  first argument must be atomic

haven't worked for me.

You won't get that desired output if you will use list. If you function will work, the output will be z 1: 3. You need to use := — David Arenburg
– David Arenburg, Commented May 28, 2014 at 11:27
@DavidArenburg. Thanks for pointing that out - was an oversight when writing. Fixed now. — mchen
– mchen, Commented May 28, 2014 at 11:33
I actually wrote an answer for your original desired output, do you still need it — David Arenburg
– David Arenburg, Commented May 28, 2014 at 11:33
Sure - thanks David. Shouldn't be too hard to tweak the final output. The important bit is how you manage to pass expressions to be evaluated in context of the data.table rather than .GlobalEnv. — mchen
– mchen, Commented May 28, 2014 at 11:34

Roland · Accepted Answer · 2014-05-28 11:40:07Z

9

This should be a better alternative to David's answer:

dynamicDT <- function(...) {
 dt[, eval(substitute(...))]
}

dynamicDT(z := x + y)
#   x y z
#1: 1 2 3

answered May 28, 2014 at 11:40

Roland

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3 Comments

David Arenburg Over a year ago

(+1) Nice one @Roland, certainly better

mchen Over a year ago

Thanks - can this handle multiple arguments such as dynamicDT(a := x + y, b := x - y)?

Roland Over a year ago

It still has to be correct data.table syntax. See David's answer for the syntax.

David Arenburg · Accepted Answer · 2014-05-28 12:09:42Z

3

You will need to use eval(parse(text = )) combination. parse will transform the string into an expression, while eval will evaluate it.

library(data.table)
dt <- data.table(x = 1, y = 2)
dynamicDT <- function(temp = "") {
  dt[, eval(parse(text = temp))]
}

In order to get your previous desired output

dynamicDT("z := x + y")
##    x y z
## 1: 1 2 3

In order to get your current desired output

dynamicDT("z = x + y")
## [1] 3

In order to parse multiple arguments you can do

dynamicDT('c("a","b") := list(x + y, x - y)')

##   x y a  b
##1: 1 2 3 -1

edited May 28, 2014 at 12:09

answered May 28, 2014 at 11:37

David Arenburg

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