如何在R中的data.table中按照两个条件选择行 [英] How do I select rows by two criteria in data.table in R

问题描述

假设我有一个data.table，并且我想选择变量x的值为b的所有行。这很容易

Let's say I have a data.table and I want to select all the rows where the variable x has a value of b. That is easy

library(data.table)
DT <- data.table(x=rep(c("a","b","c"),each=3), y=c(1,3,6), v=1:9)
setkey(DT,x)               # set a 1-column key
DT["b"]

顺便说一下，必须设置一个键，如果键没有设置为x，那么这不工作。顺便说一句，如果我设置两列作为键会发生什么？

By the way, it appears that one has to set a key, if the key is not set to x then this does not work. By the way what would happen if I set two columns as keys?

无论如何，移动，让我说，我想选择所有的行，变量x是一个或b

Anyway, moving along, lets say that I want to select all the rows where the variable x was a or b

DT["b"|"a"]

不起作用

但以下工作

DT[x=="a"|x=="b"]

b $ b

但是它使用矢量扫描和数据帧。它不使用二叉搜索。我猜对于较小的数据集，没关系。

But that uses vector scanning a la data frames. It does not use the binary search. I guess for smaller data sets it will not matter.

这是我应该做什么，还是我不了解data.table语法？

Is that what I should do or am I ignorant of data.table syntax?

。有什么例子更复杂的布尔多变量选择（或子集）过程与data.table？

And one more thing. Are there any examples of more complex Boolean multi-variable selection (or subset) procedures with data.table?

我知道我总是可以恢复使用subset（）函数，因为data.table将会像data.frame一样工作。

I know I could always revert to using the subset() function since a data.table will behave as a data.frame if it must.

推荐答案

这是一种方式，只有在我问了问题，它的工作，但我不知道它如何在基准测试之后，我的心。我目前不是在一台安装了R的计算机。我想我应该使用云实例。无论如何，我喜欢语法

Here is a way that only crossed my mind after I asked the question and it works but I do not know how it does in benchmarks. I am not currently at a computer with an installed R. I guess I should use a cloud instance. Anyway, I like the syntax

DT[c("a","b")]

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