R：强制data.table计算所有交互

Here is a data.table:

这是一个data.table:

dat = data.table(var1=rnorm(120), var2=rep(c('a','b','c'),40), var3=rep(c(1,2,3,2,1,2,1,2,2,3,1,2),10))

dat2 = dat[,list(resp = mean(var1)),by=list(var2, var3)]
dat =