Using Data Table Define Column From Character Vector

Question

I've been getting used to data.tables and just cannot seem to find the answer to something that feels so simple (or at least is with data frames).

I want to use data.table to aggregate, however, I don't always know which column to aggregate ahead of time (it takes input from the user). I want to define what column to use based off of a character vector. Here's a short example of what I want to do:

require(data.table)

myDT <- data.table(a = 1:10, b = 11:20, n1 = c("first", "second"))
aggWith <- "a"

Now I want to use the aggWith object to define what column to sum on. This does not work:

> myDT.Agg <- myDT[, .(Agg = sum(aggWith)), by = .(n1)]
Error in sum(aggWith) : invalid 'type' (character) of argument

Nor does this:

> myDT.Agg <- myDT[, .(Agg = sum(aggWith)), by = .(n1), with = FALSE]
Error in sum(aggWith) : invalid 'type' (character) of argument

This does:

myDT.Agg <- myDT[, .(Agg = sum(a)), by = .(n1)]

However, I want to be able to define which column "a" is arbitrarily based off a character vector. I've looking through ?data.table, but am just not seeing what I need. Sorry in advance if this is really simple and I'm just overlooking something.

use sum(get(aggWith)).

Arun
– Arun

2015-12-04 17:38:23 +00:00
Commented Dec 4, 2015 at 17:38 — Arun
– Arun, Commented Dec 4, 2015 at 17:38

akrun · Accepted Answer · 2015-12-04 17:37:42Z

4

We could specify the 'aggWith' as .SDcols and then get the sum of .SD

 myDT[, list(Agg= sum(.SD[[1L]] )), by = n1, .SDcols=aggWith]

If there are multiple columns, then loop with lapply

 myDT[, lapply(.SD, sum), by = n1, .SDcols= aggWith]

Another option would be to use eval(as.name

 myDT[, list(Agg= sum(eval(as.name(aggWith)))), by = n1]

answered Dec 4, 2015 at 17:37

akrun

891k38 gold badges590 silver badges700 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Frank Over a year ago

Also get, though maybe there's some reason to prefer the eval way over that?

akrun Over a year ago

@Frank I used to work with eval, though I am not sure whether this has any difference in efficiency with get.

jangorecki Over a year ago

if any of them would be faster I would expect it would be eval(as.name(

giraffehere Over a year ago

This is great stuff. Thank you!

Collectives™ on Stack Overflow

Using Data Table Define Column From Character Vector

1 Answer 1

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related