First observation by group using self-join

Question

I\'m trying to get the top row by a group of three variables using a data.table.

I have a working solution:

col1 <- c(1,1,1,1,2,2,2,2,3,3,3,3)
col         


        

Answer 1

What about:

solution2 <- data.table(data)[ , sales[1], by="store,year,month"]
> solution2
   store year month V1
1:     1 2000    12  1
2:     1 2001    12  3
3:     2 2000    12  5
4:     2 2001    12  7
5:     3 2000    12  9
6:     3 2001    12 11

I suppose you could rename that column:

data.table(data)[,fsales := sales[1],by="store,year,month"]