Transpose/reshape rows in columns with given conditions

Question

I have the following data:

request   user   group
1         1      1
4         1      1
7         1      1
5         1      2
8         1      2
1         2              


        

Answer 1

aggregate gets you very close:

dat <- data.frame(
    user = c(1,1,1,2,2,3,3,3,4), 
    group = c(1,1,1,1,1,2,2,2,2), 
    request = c(1,4,7,5,8,1,4,7,9)
)

aggregate(request~group + user, dat, FUN=c)

  group user request
1     1    1 1, 4, 7
2     1    2    5, 8
3     2    3 1, 4, 7
4     2    4       9