r - 按ID分组，新表的每个元素都是一个vector

Question

我有一张这样的 table

data.table(ID = c(1,2,3,4,5,6), 
         R = c("s","s","n","n","s","s"), 
         S = c("a","a","a","b","b","b"))

我正在努力得到这个结果

      a     b
s   1, 2      5, 6
n     3         4

data.table 中是否有任何选项可以做到这一点？

Answer 1

这是一个使用普通旧 data.table 语法的替代方法:

DT[,lapply(split(ID,S),list),by=R]

# or...

DT[,lapply(split(ID,S),toString),by=R]