如何调用一个返回data.table中多个行和列的函数？

Question

我想在data.table中调用一个函数来计算一组汇总统计信息,如下所示:

summ.stats <- function(vec) {
    list(
         Min = min(vec),
         Mean = mean(vec),
         S.D. = sd(vec),
         Median = median(vec),
         Max = max(vec))
}

我想把它j称为data.table:

DT <- data.table(a=c(1,2,3,1,2,3),b=c(1,4,3,2,1,4),c=c(2,3,4,5,2,1))

DT[, summ.stats(b), by=a]

这很好,我得到:

   a Min Mean      S.D. Median Max
1: 1   1  1.5 0.7071068    1.5   2
2: 2   1  2.5 2.1213203    2.5   4
3: 3   3  3.5 0.7071068    3.5   4

但我有兴趣将多个变量传递给summ.stats.例如:

DT[, summ.stats(b, c), by=a]

我希望得到类似的东西:

   a Var Min Mean      S.D. Median Max
1: 1   b   1  1.5 0.7071068    1.5   2
2: 2   b   1  2.5 2.1213203    2.5   4
3: 3   b   3  3.5 0.7071068    3.5   4
4: 1   c   2  3.5 2.1213203    3.5   5
5: 2   c   2  2.5 0.7071068    2.5   3
6: 3   c   1  2.5 2.1213203    2.5   4

做这个的最好方式是什么？

Answer 1

或者,您可以按如下方式修改函数:

summ.stats <- function(vec) {
    list(
        Var = names(vec),
         Min = sapply(vec, min),
         Mean = sapply(vec, mean),
         S.D. = sapply(vec, sd),
         Median = sapply(vec, median),
         Max = sapply(vec, max))
}

DT[, summ.stats(.SD), by=a] # no need for as.list(.SD) as Roger mentions
   a Var Min Mean      S.D. Median Max
1: 1   b   1  1.5 0.7071068    1.5   2
2: 1   c   2  3.5 2.1213203    3.5   5
3: 2   b   1  2.5 2.1213203    2.5   4
4: 2   c   2  2.5 0.7071068    2.5   3
5: 3   b   3  3.5 0.7071068    3.5   4
6: 3   c   1  2.5 2.1213203    2.5   4