Dear all,
I have the following  R dataframe:

  set.seed(11)
   (df <- data.frame(ID=rep(1:10,1:10),a=factor(sample(1:4,55,rep=T))    ))
   where ID is an identification code.
   I need to create a new variable "b" in which I would paste the full group
of "a" variable according to ID variable.
   For example for ID=2 "b" should be 13, for ID=3 "b" should be 114, and
for ID=4 should be 1241.
  How can I do that ?
  Thank you for any advices,
  Regards,
   Massimiliano Tripoli

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