Adding columns to a data table

Question

I have a data.frame (or a matrix or any other tabular data structure object for that matter):

df = data.frame(field1 = c(1,1,1),field2 = c(2,2,2),field3 = c(         


        

Answer 1

I think you want cbind

cbind(dt, df[, 1:2])
# fieldX field1 field2
# 1      x      1      2
# 2      x      1      2
# 3      x      1      2

Answer 2

You can use cbind:

cbind(dt, df[fields])

However, the most efficient way is still probably going to be to use data.table's assign by reference:

dt[, (fields) := df[fields]]