使用字符串访问data.table列 [英] Access data.table columns with strings

问题描述

很抱歉，这个问题很明显地表明我通常在Python / pandas中工作，但我对此一无所知。如何使用字符串选择 data.table 列？

Apologies for a question that probably makes it obvious that I usually work in Python/pandas, but I'm stuck with this. How do I select a data.table column using a string?

dt$"string"
dt$as.name("string")
dt$get("string")

我确定这是非常简单的，但我没有。

I'm sure this is super simple, but I'm not getting it. Any help is greatly appreciated!

在以下一些有用的评论和提示之后，我想ve缩小了问题的范围，并提供了可重复的示例。考虑：

After some of the helpful comments and tips below, I think I've narrowed down the problem a bit and have a reproducible example. Consider:

dt = data.table(ID = c("a","a","a","b","b","b"), col1=rnorm(6), col2=rnorm(6)*100)

并假设我们要将 col2 中的值分配给 col1 。正如我在下面了解到的那样， data.table 的语法将是 dt [，col1：= col2] ，干净简单。当 j 参数中的一个（或两个）变量是字符串时，问题就开始了。我发现了以下内容：

And assume we want to assign the values in col2 to col1. As I've learned below, the data.table syntax for this would be dt[,col1:=col2], clean and simple. The problems start when one (or both) of the variables in the j argument are strings. I found the following:

dt [， col1：= col2] 可以正常工作

dt [， col1：= col2] 失败（尝试分配字符 col2 到对偶向量 col1

dt[, "col1":="col2"] fails as expected (tries to assign the character col2 to the double vector col1

dt [ ， col1：= get（ col2）] 正常工作

dt [，get（ col1）] 返回预期的 col1

但是： dt [，get（ col1）：= col2] 或其他分配失败。

but: dt[, get("col1"):=col2] or any other assignment fails.

某些情况：这样做的原因是我在循环中构造字符串，以访问大量都命名为 colname_colnumber 的列，即我遍历 colname 和 colnumber 来访问列 paste0（colname，colnumber）。

Some context: the reason for doing this is that I'm constructing strings in a loop, to access a larger number of columns that are all named colname_colnumber, i.e. I loop over colname and colnumber to then access column paste0(colname,colnumber).

推荐答案

您可以将 get（）用作 j 参数使用单括号：

You can use get() as the j argument using single brackets:

library(data.table)
dt <- data.table(iris)
dt[, get("Species")]

结果：

[1] setosa     setosa     setosa     setosa     setosa     setosa .....

您也可以在双括号运算符内直接使用字符串，如下所示：

You can also use a string directly inside the double bracket operator, like this:

dt[["Species"]]

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