R中的过滤表

我想这样过滤我的表

输入表：

x1    x2    x3
A    B     x 
A    B     y
A    B     z
C    D     u
C    D     v

x1    x2    x3
A     B     x,y,z
C     D     u,v

输出表：

x1    x2    x3
A    B     x 
A    B     y
A    B     z
C    D     u
C    D     v

x1    x2    x3
A     B     x,y,z
C     D     u,v

可再现数据

df <- data.frame(
  x1 = c("A", "A", "A", "C", "C"), 
  x2 = c("B", "B", "B", "D", "D"), 
  x3 = c("x", "y", "z", "u", "v"),
  stringsAsFactors = FALSE
)

---

x3的预期结构是什么？您尝试过什么？如果您更喜欢列表列而不是字符串列，请将FUN参数更改为：FUN=functionx listx