Add two columns simultaneasouly to a data.table in R

Question

I have this data.table:

DT <- data.table(num=c(1,4,6,7,8,12,13, 15), let=rep(c("A","B"), each=4))

Displayed like this:

   num let
1:   1   A
2:   4   A
3:   6   A
4:   7   A
5:   8   B
6:  12   B
7:  13   B
8:  15   B

And I define this function:

f<-function(x){return(c(x+1,x+2))}

I want to get this:

   let V1 V2
1:   A  2  3
2:   A  5  6
3:   A  7  8
4:   A  8  9
5:   B  9 10
6:   B 13 14
7:   B 14 15
8:   B 16 17

I run this code:

DT[,list(f(num)),by=let]

and

DT[,f(num),by=let]

But I get this:

I would add that I do not want to modify the function f at all.

eddi · Accepted Answer · 2014-06-19 15:04:46Z

8

DT[, as.data.table(matrix(f(num), ncol=2)), by=let]
#   let V1 V2
#1:   A  2  3
#2:   A  5  6
#3:   A  7  8
#4:   A  8  9
#5:   B  9 10
#6:   B 13 14
#7:   B 14 15
#8:   B 16 17

Of course it would be preferable and more efficient to simply change the function, since its design is the problem and not data.table. Your function will always return an atomic vector, regardless of if it is used inside data.table.

f <- function(x) list(x+1, x+2)
DT[, f(num), by=let]

edited Jun 19, 2014 at 15:04

eddi

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answered Jun 19, 2014 at 7:39

Roland

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2 Comments

Colonel Beauvel Over a year ago

Oh thanks, modifying the function is the best way! I dunno if it possible to change the name of V1 and V2 by the way.

Roland Over a year ago

f <- function(x) list(a=x+1, b=x+2) or if you prefer not modifying the function DT[, setNames(f(num), c("a", "b")), by=let].

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