第二列的data.table索引

Question

我有两个分类列(A,B)和数字列(C).我想获得A的值,其中C是B定义的组的最大值.我正在寻找一个data.table解决方案.

library(data.table)

dt <- data.table( A = c("a","b","c"), 
                  B = c("d","d","d"), 
                  C = c(1,2,3))
dt
   A B C
1: a d 1
2: b d 2
3: c d 3

# I want to find the value of A for the maximum value
# of C when grouped by B
dt[,max(C), by=c("B")]
   B V1
   1: d  3

#how can I get the A column, value = "c"

Answer 1

另一种选择是排序C并仅提取唯一的B组.对于具有多个组的大数据集,这应该更快,因为它不计算每组的最大值,而是仅排序一次

unique(dt[order(-C)], by = "B")
#    A B C
# 1: c d 3