您非常亲密。应该这样做：

You are very close. This should do it:

row_numbers <- DT[exclude==0 | y==5, .I[{
    x <- rep(which(y==5), each=length(idx)) + idx 
    x[x>0 & x<=.N]
  }], by=grp]$V1
DT[row_numbers]