在data.table中创建组

Question

如何在data.table中创建组？

我有一个data.table如下：

library(data.table)
dt <- data.table(V1=c("A","B","C"))

现在我想为V1的每个实现添加一组字符串：

myGroup <- c("d", "e", "f")

result <- data.table(V1=c("A","A","A","B","B","B","C","C","C"),
                     V2=c("d","e","f","d","e","f","d","e","f"))

非常感谢您的投入。

Answer 1

out <- dt[, .(V2 = myGroup), by = V1]

#    V1 V2
# 1:  A  d
# 2:  A  e
# 3:  A  f
# 4:  B  d
# 5:  B  e
# 6:  B  f
# 7:  C  d
# 8:  C  e
# 9:  C  f

all.equal(out, result)
# [1] TRUE

修改
根据Per @ Frank的评论，您可以等效且更具惯用性：dt[, CJ(V1, myGroup)]（CJ代表（C）ross（J）oin）。