Applying a function to each row of a data.table

Question

I looking for a way to efficiently apply a function to each row of data.table. Let\'s consider the following data table:

library(data.table)
library(stringr)         


        

Answer 1

How about :

x
   a     b
1: 1 12 13
2: 2 14 15
3: 3 16 17
4: 1 18 19

x[,list(a=rep(a,each=2), V1=unlist(strsplit(b," ")))]
   a V1
1: 1 12
2: 1 13
3: 2 14
4: 2 15
5: 3 16
6: 3 17
7: 1 18
8: 1 19

Generalized solution given comment :

x[,{s=strsplit(b," ");list(a=rep(a,sapply(s,length)), V1=unlist(s))}]