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, May 28 '14 at 11:27
@DavidArenburg. Thanks for pointing that out - was an oversight when writing. Fixed now. — mchen, May 28 '14 at 11:33
I actually wrote an answer for your original desired output, do you still need it — David Arenburg, May 28 '14 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, May 28 '14 at 11:34

score 9 · Accepted Answer · answered May 28 '14 at 11:40

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 '14 at 11:40

Roland

127,288
10
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288

(+1) Nice one @Roland, certainly better – David Arenburg May 28 '14 at 11:41
Thanks - can this handle multiple arguments such as `dynamicDT(a := x + y, b := x - y)`? – mchen May 28 '14 at 11:42
It still has to be correct data.table syntax. See David's answer for the syntax. – Roland May 28 '14 at 12:31

David Arenburg · Answer 2 · 2014-05-28T12:09:42.020

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

How to use data.table inside a function?

Attempts:

2 Answers2