How to reference a variable which is not in data.table header?

Asked Nov 19 '18 at 18:06

Active Nov 20 '18 at 20:40

Viewed 60 times

I have a data.table DT, which contains 2 columns.

library(data.table)
DT = data.table(a = c(1, 2), b = c(2, 3))

I have a variable, named i

i <- "A"

Now I want to use i to determine how to create a new column in data.table. If i == "A", I want to create a new column, which is a*b.

However, if I use DT[ , NEW := ifelse(i == "A", a*b, a)]

The result is in below, which is not what I want.

   a b NEW
1: 1 2   2
2: 2 3   2

The result that I am looking for is

   a b NEW
1: 1 2   2
2: 2 3   6

I know if I generate a new column named i, I can get what I want.

DT[ , i := i]
DT[ , NEW := ifelse(i == "A", a*b, a)]
   a b i NEW
1: 1 2 A   2
2: 2 3 A   6

Is there a simple way to reference variable i, without generating a new column named i?

Thanks

edited Nov 20 '18 at 20:40

Henrik

asked Nov 19 '18 at 18:06

SQLNewUser

use `if`/`else` not `ifelse` – MichaelChirico Nov 19 '18 at 18:20
https://stackoverflow.com/questions/17252905/else-if-vs-ifelse – MichaelChirico Nov 19 '18 at 18:20
also may be a situation where you consider using `switch`; see `?switch` – MichaelChirico Nov 19 '18 at 18:21
3

Or raise 'b' to the power of your logical test, `DT[ , NEW := a * b^(i == "A")]` – Henrik Nov 19 '18 at 18:44
@MichaelChirico Thank you! Simple if/else works. – SQLNewUser Nov 19 '18 at 21:00
@Henrik Thank you. Your solution also works. – SQLNewUser Nov 19 '18 at 21:01

0 Answers0