Add column to empty data.table in R

Question

For adding a new column to an existing empty data.table (version 1.8.6) there seems to be no way to do it without being warned.

Example:

dt<-old.table[0] dt[,new_column:=""] 

This produces the warning:

In '[.data.table'(dt, , ':='(new_column,"")):     Supplied 1 items to be assigned to 0 items of column 'new_column' (1 unused) 

Is there a way to add a new column without warnings?

Answer 1

Good question. Assign an empty character vector (character()) rather than a length 1 character vector ("").

> DT = data.table(a=1:3,b=4:6) > DT2 = DT[0] > DT2 Empty data.table (0 rows) of 2 cols: a,b > DT2[,newcol:=character()]    # no warning > DT2 Empty data.table (0 rows) of 3 cols: a,b,newcol > sapply(DT2,class)           a           b      newcol    "integer"   "integer" "character"  

Btw, ""[0] is another way to create a 0 length character vector; 7 characters less typing than character() but possibly less readable, depending on your preference.