data.table “关键索引"或“组计数器" [英] data.table "key indices" or "group counter"

问题描述

在 data.table 上创建键后:

After creating a key on a data.table:

set.seed(12345)
DT <- data.table(x = sample(LETTERS[1:3], 10, replace = TRUE),
                 y = sample(LETTERS[1:3], 10, replace = TRUE))
setkey(DT, x, y)
DT
#       x y
#  [1,] A B
#  [2,] A B
#  [3,] B B
#  [4,] B B
#  [5,] C A
#  [6,] C A
#  [7,] C A
#  [8,] C A
#  [9,] C C
# [10,] C C

我想获得一个整数向量，为每一行提供相应的键索引".我希望下面的预期输出(i 列)将有助于澄清我的意思:

I would like to get an integer vector giving for each row the corresponding "key index". I hope the expected output (column i) below will help clarify what I mean:

#       x y i
#  [1,] A B 1
#  [2,] A B 1
#  [3,] B B 2
#  [4,] B B 2
#  [5,] C A 3
#  [6,] C A 3
#  [7,] C A 3
#  [8,] C A 3
#  [9,] C C 4
# [10,] C C 4

我考虑过使用类似 cumsum(!duplicated(DT[, key(DT), with = FALSE])) 的东西，但我希望有更好的解决方案.我觉得这个向量可能是表格内部表示的一部分，也许有办法访问它?即使不是这样，你有什么建议?

I thought about using something like cumsum(!duplicated(DT[, key(DT), with = FALSE])) but am hoping there is a better solution. I feel this vector could be part of the table's internal representation, and maybe there is a way to access it? Even if it is not the case, what would you suggest?

推荐答案

更新:从v1.8.3，你可以简单地使用内置的特殊.GRP:p>

Update: From v1.8.3, you can simply use the inbuilt special .GRP:

DT[ , i := .GRP, by = key(DT)]

查看历史以获取较早的答案.

See history for older answers.

这篇关于data.table “关键索引"或“组计数器"的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！