与数据表的非联接 [英] non-joins with data.tables
问题描述
我对非联接的 data.table idiom有一个问题，源自Iterator的问题。这里有一个例子：
library（data.table）
dt1< - data.table （A1 = letters [1:10]，B1 = sample（1：5,10，replace = TRUE））
dt2
setkey（dt1，A1）
setkey（dt2，A2）
data.table 看起来像这样
> dt1> dt2
A1 B1 A2 B2
[1，] a 1 [1，] a 2
[2，] b 4 [2，] b 5
[3，] c 2 [3，] c 2
[4，] d 5 [4，] d 1
[5，] e 1 [5，] e 1
[6，] f 2 [ 6，] k 5
[7，] g 3 [7，] l 2
[8，] h 3 [8，] m 4
[9，] i 2 [ ] n 1
[10，] j 4 [10，] o 1
dt2 中的哪些行在 dt1 中具有相同的键，请设置，其中选项 TRUE ：
dt1 [dt2，which = TRUE]
[1] 1 2 3 4 5 NA NA NA NA NA
$ b b
Matthew在 [-dt1 [dt2，d3，d3，d3，d3，d3，d4]，则非加入成语
$ 7/ which = TRUE]]
子集 dt1 指向那些没有出现在 dt2 中的索引的行。在我的机器上 data.table v1.7.1我收到一个错误：
`[.default`（x [[s]]，irows）中的错误：只有0可能与负下标混合
而是使用 nomatch = 0 选项，非加入工作
> dt1 [-dt1 [dt2，which = TRUE，nomatch = 0]]
A1 B1
[1，] f 2
[2，] g 3
[3， h 3
[4，] i 2
[5，] j 4
这是预期的行为吗？
据我所知，这是基础R的一部分。
＃this works
（1：4）[c（-2，-3）]
＃但是这给了你上面描述的相同的错误
（1：4）[c（-2，-3，NA）]
＃错误在（1：4）[c 3，NA）]：
＃只有0可以与负下标混合
这是我最好的猜测，为什么这是预期的行为：
从他们处理 NA 在别处的方式（例如通常默认为 na .rm = FALSE ），看起来R的设计师将 NA 的视为携带重要信息，并且没有一些明确的指令这样做。 （幸运的是，设置 nomatch = 0 给你一个干净的方式来传递指令！）
，设计者的偏好可能解释为什么 NA '被接受用于正索引，而不是用于负索引：
＃正索引：因为返回值保留了关于NA的
（1：4）的信息[c（2,3，NA）]
＃负索引：不工作，因为它不能轻易保留这样的信息
（1：4）[c（-2，-3，NA）]
I have a question on the data.table idiom for "non-joins", inspired from Iterator's question. Here is an example:
library(data.table)
dt1 <- data.table(A1=letters[1:10], B1=sample(1:5,10, replace=TRUE))
dt2 <- data.table(A2=letters[c(1:5, 11:15)], B2=sample(1:5,10, replace=TRUE))
setkey(dt1, A1)
setkey(dt2, A2)
The data.tables look like this
> dt1 > dt2
A1 B1 A2 B2
[1,] a 1 [1,] a 2
[2,] b 4 [2,] b 5
[3,] c 2 [3,] c 2
[4,] d 5 [4,] d 1
[5,] e 1 [5,] e 1
[6,] f 2 [6,] k 5
[7,] g 3 [7,] l 2
[8,] h 3 [8,] m 4
[9,] i 2 [9,] n 1
[10,] j 4 [10,] o 1
To find which rows in dt2 have the same key in dt1, set the which option to TRUE:
> dt1[dt2, which=TRUE]
[1] 1 2 3 4 5 NA NA NA NA NA
Matthew suggested in this answer, that a "non join" idiom
dt1[-dt1[dt2, which=TRUE]]
to subset dt1 to those rows that have indexes that don't appear in dt2. On my machine with data.table v1.7.1 I get an error:
Error in `[.default`(x[[s]], irows): only 0's may be mixed with negative subscripts
Instead, with the option nomatch=0, the "non join" works
> dt1[-dt1[dt2, which=TRUE, nomatch=0]]
A1 B1
[1,] f 2
[2,] g 3
[3,] h 3
[4,] i 2
[5,] j 4
Is this intended behavior?
As far as I know, this is a part of base R.
# This works
(1:4)[c(-2,-3)]
# But this gives you the same error you described above
(1:4)[c(-2, -3, NA)]
# Error in (1:4)[c(-2, -3, NA)] :
# only 0's may be mixed with negative subscripts
The textual error message indicates that it is intended behavior.
Here's my best guess as to why that is the intended behavior:
From the way they treat NA's elsewhere (e.g. typically defaulting to na.rm=FALSE), it seems that R's designers view NA's as carrying important information, and are loath to drop that without some explicit instruction to do so. (Fortunately, setting nomatch=0 gives you a clean way to pass that instruction along!)
In this context, the designers' preference probably explains why NA's are accepted for positive indexing, but not for negative indexing:
# Positive indexing: works, because the return value retains info about NA's
(1:4)[c(2,3,NA)]
# Negative indexing: doesn't work, because it can't easily retain such info
(1:4)[c(-2,-3,NA)]
这篇关于与数据表的非联接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！
