Repeat one column values by another column values in R data frame

Question

I have following data frame:

v1 v2
1 a,b,a
2 a,c
3 b,b
4 a,c,b,c

How can I use this to have following data frame?

v1 v2 
1 a   
1 b
1 a
2 a   
2 c   
3 b
3 b   
4 a   
4 c   
4 b
4 c

Answer 1

Try cSplit

 library(data.table)
 library(devtools)
  source_gist(11380733) ##

 cSplit(df, 'v2', ",", 'long')
 #    v1 v2
 #1:  1  a
 #2:  1  b
 #3:  1  a
 #4:  2  a
 #5:  2  c
 #6:  3  b
 #7:  3  b
 #8:  4  a
 #9:  4  c
#10:  4  b
#11:  4  c