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)

我想选择一个子集通过使用存储相关变量名称的对象以编程方式（动态）对变量进行编程。例如，我要选择存储在变量 keep中的两列 V1和 V3

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")

如果我们选择保留 data.frame中的列，则可以使用以下内容：

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。我以为data.frame和data.table与这种行为相同，但是显然它们不是。有人能够建议正确的语法吗？

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

常见问题解答1.1、1.2和2.17 。

有些可能性：

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

DF [c（'V1'，'V3'）] 起作用的原因 data.frame 是否包含在中？`[.data.frame`

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

数据帧可以用几种模式索引。当使用 [和 [[时，
具有单个向量索引（ x [i] 或 x [[i]] ），它们对数据框
进行索引，就好像它是一个列表一样。在这种用法中，将忽略 drop 参数，并带有
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.

---

来自 data.table 1.10.2 ，则可以使用。 。以编程方式子集化列的前缀：

---

From data.table 1.10.2, you may use the .. prefix when subsetting columns programmatically:

当 j 是前缀为的符号。它将在调用范围中查找，其值将用作列名或数字[...]这是实验性的。 p>


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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