a simple loop with data.table

Question

I want to do a simple loop using data.table. I have 20 dichotomous (0,1) variables (from var_1 to var_20) and I would like to do a loop for this:

dat[var_1==1, newvar:=1]
dat[var_2==1, newvar:=2]
dat[var_3==1, newvar:=3]
...
dat[var_20==1, newvar:=21]

My main problem is I don't know how specify i (i.e. var_1==1, var_2==2...) using a loop. Below a short example:

var_1  <- c(1, rep(0,9))
var_2  <- c(0,1, rep(0,8))
var_3  <- c(0,0,1, rep(0,7))
dat  <- data.table(var_1, var_2, var_3)

dat[var_1==1, newvar:=1]
dat[var_2==1, newvar:=2]
dat[var_3==1, newvar:=3]

Any ideas about how to do this with a loop? Thanks!

Answer 1

To take advantage of data.table class it is better to set key.

dat[ ,newvar:= NA_integer_]
for(i in ncol(dat)) {
 setkeyv(dat, names(dat)[i])
 dat[J(1), newvar:=i]
}

Answer 2

Something like this will work.

nams <- names(dat)
for(n in seq_along(nams)){
  nam <- nams[n] 
  char <- sprintf('%s==1',nam)
  dat[eval(parse(text=char)), newvar := n]
}
dat
var_1 var_2 var_3 newvar
1:     1     0     0      1
2:     0     1     0      2
3:     0     0     1      3
4:     0     0     0     NA
5:     0     0     0     NA
6:     0     0     0     NA
7:     0     0     0     NA
8:     0     0     0     NA
9:     0     0     0     NA
10:    0     0     0     NA