R Aggregate by two factors

Question

Suppose I have a data.table x :

 x <- data.table(Motif = c("A","A","B","B","B","C"),
                 Strain = c(1,1,1,2,2,3),
                 IPDRatio = c(1:6))

How would I find the mean of the IPDRatio column by collapsing the entire data.table by factors Motif and Strain ?

Desired output:

   Motif Strain IPDRatio
1:     A      1      1.5
2:     B      1        3
3:     B      2      4.5
4:     C      3        6

Answer 1

这应该工作

x[, mean(IPDRatio), list(Motif, Strain)]