在 data.table 对象中显示行名 [英] Display row names in a data.table object

问题描述

参考:在尝试回答时 这个非常基本的问题，我突然意识到我无法在 data.table 对象中显示 rownames

Reference: While trying to answer this very basic question, I suddenly realized that I wasn't able to display rownames in a data.table object

玩具示例

library(data.table)
DT <- data.table(A = letters[1:3])
DT
##    A
## 1: a
## 2: b
## 3: c
row.names(DT) <- 4:6
row.names(DT)
## [1] "4" "5" "6" # seem to work

或

rownames(DT) <- 7:9
rownames(DT)
## [1] "7" "8" "9" # seems to be ok too

但是当显示数据本身时，行名保持不变

But when displaying the data itself, row names remains unchanged

DT
##    A
## 1: a
## 2: b
## 3: c

我会假设 data.table 出于效率目的而忽略不必要的属性，但 attributes 似乎不同意

I would assume data.table ignores unnecessary attributes for efficiency purposes, but attributes seem to disagree

attributes(DT)
# $names
# [1] "A"
# 
# $row.names
# [1] 7 8 9
# 
# $class
# [1] "data.table" "data.frame"
# 
# $.internal.selfref
# <pointer: 0x0000000000200788>

推荐答案

这或多或少是逐字逐句的.

This is more or less verbatim from comments.

data.table 不支持行名.这是故意的，因为行名是一个糟糕的设计选择，因为它们比列使用起来麻烦得多(尤其是在 data.table 中，列比在data.frame)并且只是可以表示的数据列类型的一个子集(回想一下 data.frame 中的行名只是一个字符向量，而列可以是任何东西).

data.table doesn't support row names. This is intentional, as row names are a bad design choice, because they are far more cumbersome to use than columns (and especially so in data.table, where columns are so much easier to deal with than in data.frame) and are only a subset of what kind of data columns can represent (recall that row names in data.frame are a character vector only, whereas columns can be anything).

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