Retrieve the column name inside the lapply with .SD

Question

I would like to apply a function to all column in data.table. Hence, I use .SD with lapply. But, inside lapply I cannot retrieve the column of my table.

For instance

x = data.table(a=1:10, b=10:1, id=1:5)
x[,lapply(.SD,function(t){t*id}),.SDcols=c(1,2)]
Error in ..FUN(a) : object 'id' not found

I do the following:

x[,lapply(.SD,function(t){t*x$id}),.SDcols=c(1,2)]

Can we do better?

Answer 1

just remove .SDcols=c(1,2). that removes the third column (id)

 > x[,lapply(.SD,function(t){t*id})]
     a  b id
 1:  1 10  1
 2:  4 18  4
 3:  9 24  9
 4: 16 28 16
 5: 25 30 25
 6:  6  5  1
 7: 14  8  4
 8: 24  9  9
 9: 36  8 16
10: 50  5 25

to not have the id, all the following will work:

x[,lapply(.SD[,list(a,b)], `*`, id)]

x[,lapply(.SD[,-3], `*`, id)]

x[,lapply(.SD, `*`,id)][, list(a,b)]