在`by`参数中使用字符向量 [英] Use a character vector in the `by` argument

问题描述

在R的 data.table 包中，有没有一种方法可以使用字符向量在中由by分配计算的参数？

Within the data.table package in R, is there a way in order to use a character vector to be assigned within the by argument of the calculation?

以下是使用mtcars从中得到的期望输出示例：

Here is an example of what would be the desired output from this using mtcars:

 mtcars <- data.table(mtcars)
 ColSelect <- 'cyl' # One Column Option
 mtcars[,.( AveMpg = mean(mpg)), by = .(ColSelect)] # Doesn't work

 # Desired Output 
    cyl   AveMpg
 1:   6 19.74286
 2:   4 26.66364
 3:   8 15.10000

我知道可以在 j

I know that this is possible to use assigning column names in j by enclosing the vector around brackets.

 ColSelect <- 'AveMpg' # Column to be assigned for average mpg value
 mtcars[,(ColSelect):= mean(mpg), by = .(cyl)]
 head(mtcars)

    mpg cyl disp  hp drat    wt  qsec vs am gear carb   AveMpg
1: 21.0   6  160 110 3.90 2.620 16.46  0  1    4    4 19.74286
2: 21.0   6  160 110 3.90 2.875 17.02  0  1    4    4 19.74286
3: 22.8   4  108  93 3.85 2.320 18.61  1  1    4    1 26.66364
4: 21.4   6  258 110 3.08 3.215 19.44  1  0    3    1 19.74286
5: 18.7   8  360 175 3.15 3.440 17.02  0  0    3    2 15.10000
6: 18.1   6  225 105 2.76 3.460 20.22  1  0    3    1 19.74286

是否有建议为了实现此目的而在 by 参数中添加什么？

Is there a suggestion as to what to put in the by argument in order to achieve this?

推荐答案

来自中？data.table 部分，它表示 by 接受：

• 包含逗号分隔列名的单个字符串（空格很重要，因为列名即使在开头或结尾也可能包含空格
  
  ）：例如DT [，sum（a） ，by = x，y，z]


• 列名称的字符向量：例如DT [，sum（a），by = c（ x， y ）]

是的，您可以在@cccmir的回复中使用答案。您也可以使用 c（），如@akrun所述，但这似乎有点多余，除非您想要多个列。

So yes, you can use the answer in @cccmir's response. You can also use c() as @akrun mentioned, but that seems slightly extraneous unless you want multiple columns.

不能使用。（）语法的原因是在 data.table 中。 ）是 list（）的别名。并且根据 by 的相同帮助， list（）语法要求使用列名的表达式-而不是字符串。

The reason you cannot use .() syntax is that in data.table .() is an alias for list(). And according to the same help for by the list() syntax requires an expression of column names - not a character string.

如果要使用多个变量并将名称作为字符传递，请参考 by 帮助中的示例。您可以这样做：

Going off the examples in the by help if you wanted to use multiple variables and pass the names as characters you could do:

1. mtcars [，..（AveMpg = mean（mpg）），by = cyl， am]


2. mtcars [，..（AveMpg = mean（mpg）），by = c（ cyl， am ）]

1. mtcars[,.( AveMpg = mean(mpg)), by = "cyl,am"]
2. mtcars[,.( AveMpg = mean(mpg)), by = c("cyl","am")]

这篇关于在`by`参数中使用字符向量的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！