r - data.table 默认外部连接与 setkey，计算行(列出键的频率)

Question

我有两个数据表。我想用一个中的键来计算另一个中的出现次数。

DT1 <- data.table(A = c("v1","v2","v1","v2","v3","v4","v5","v6","v1","v2"),
                  B = c(1,11,111,2,22,222,3,33,333,4))
DT2 <- data.table(C = c("v1","v3","v99"))

setkey(DT1,A)
setkey(DT2,C)

使用 DT2 在 DT1 中查找行返回外连接。

nrow(DT1[DT2,]) #returns 5 (v1*3, v3*1, v99) "v99" not in DT2
DT1[DT2,]

    A   B
1:  v1   1
2:  v1 111
3:  v1 333
4:  v3  22
5: v99  NA

是否可以使用相同的语法强制左连接？

Answer 1

I want to use the keys in one to count the occurrences in another.

DT1[DT2, .N, on=c(A="C"), by=.EACHI]

     A N
1:  v1 3
2:  v3 1
3: v99 0