如何以编程方式在 data.table 中选择列? [英] How to select columns programmatically in a data.table?

问题描述

我有以下 data.table (DT):

I have the following data.table (DT):

DT <- data.table(V1 = 1:3, V2 = 4:6, V3 = 7:9)

我想通过使用存储相关变量名称的对象以编程方式(动态)选择变量的子集.例如，我想选择存储在变量keep"中的V1"和V3"两列

I would like to select a subset of the variables programmatically (dynamically), by using an object where the relevant variable names are stored. For example, I want to select the two columns "V1" and "V3" stored in a variable "keep"

keep <- c("V1", "V3")

如果我们要从 data.frame 中选择保留"列，则可以使用以下方法:

If we were to select the "keep" columns from a data.frame, the following would work:

DT[keep]

不幸的是，当这是一个 data.table 时，这不起作用.我认为 data.frame 和 data.table 与这种行为相同，但显然它们不是.有人能就正确的语法提出建议吗?

Unfortunately, this is not working when this is a data.table. I thought the data.frame and data.table are identical with this kind of behavior, but apperently they aren't. Anybody able to advise on the correct syntax?

推荐答案

这在 常见问题解答 1.1、1.2 和 2.17.

一些可能性:

DT[, keep, with = FALSE]
DT[, c('V1', 'V3'), with = FALSE]
DT[, c(1, 3), with = FALSE]
DT[, list(V1, V3)]

DF[c('V1','V3')] 像 data.frame 一样工作的原因在 ?`[.data.frame`

The reason DF[c('V1','V3')] works as it does for a data.frame is covered in ?`[.data.frame`

可以在多种模式下对数据帧进行索引.使用 [ 和 [[ 时使用单个向量索引(x[i] 或 x[[i]])，它们索引数据帧好像它是一个列表.在这种用法中，一个 drop 参数被忽略，带有一个警告.

Data frames can be indexed in several modes. When [ and [[ are used with a single vector index (x[i] or x[[i]]), they index the data frame as if it were a list. In this usage a drop argument is ignored, with a warning.

<小时>

来自 data.table 1.10.2，您可以在以编程方式设置列子集时使用 .. 前缀:

当 j 是一个以 .. 为前缀的符号时，它将在调用范围内查找，其值被视为列名或数字 [...]是实验性的.

When j is a symbol prefixed with .. it will be looked up in calling scope and its value taken to be column names or numbers [...] It is experimental.

因此:

DT[ , ..keep]
#    V1 V3
# 1:  1  7
# 2:  2  8
# 3:  3  9

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