CODEWITHSUNDEEP
rdata.table
user1841430
user1841430

Reputation: 261

Add column to empty data.table in R

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?

Upvotes: 26

Views: 22441

Answers (2)

Beasterfield
Beasterfield

Reputation: 7113

Just as an addition how to add an empty character column, when the data.table has an arbitrary number of rows (including 0):

DT2[ ,newcol:=character(.N) ]

Upvotes: 9

Matt Dowle
Matt Dowle

Reputation: 59602

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.

Upvotes: 35

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