使用列名的字符向量访问 data.table 中的列 [英] Accessing columns in data.table using a character vector of column names

问题描述

假设我有一个 R 数据表:

Suppose I have an R data.table:

 DT = data.table(x=rep(c("a","b","c"),each=3), y=c(1,3,6), v=1:9)

我有一个列名的字符向量，我想提取或者更一般地对其进行操作:

and I have a character vector of column names that I would like to extract, or more generally operate on:

cols = c("x","y")

例如，如何使用cols来生成等价的

For example, how can I use cols to generate the equivalent of

 DT[,lapply(.SD[,list(x,y)], min) ]

有没有办法使用 cols 向量指定 list(x,y)?

Is there a way to specify the list(x,y) using the cols vector?

推荐答案

您可以使用 data.table 语法 .. 来查找一级"(如在 Unix 终端中)为变量:

You can use the data.table syntax .. which "looks up one level" (as in the Unix terminal) for the variable:

> all.equal(DT[,list(x,y)], DT[, ..cols])
[1] TRUE
> all.equal(DT[,.SD[,list(x,y)][min(v)]], DT[,.SD[ ,min(v)], .SDcols = cols])
[1] TRUE

FAQ 1.6 下的更多详细信息我相信:http://datatable.r-forge.r-project.org/datatable-faq.pdf

More details under FAQ 1.6 I believe: http://datatable.r-forge.r-project.org/datatable-faq.pdf

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