如何在 data.table 中引用以数字开头的列名 [英] How to reference column names that start with a number, in data.table

问题描述

如果data.table中的列名是数字+字符的形式，例如:4PCS,5Y 等，如何在 x[i,j] 中将其引用为 j 以便将其解释为不带引号的列名.

If the column names in data.table are in the form of number + character, for example: 4PCS, 5Y etc, how could this be referenced as j in x[i,j] so that it is interpreted as an unquoted column name.

我认为这将解决我原来的问题.我想在data.table"中添加几列，格式为 number + character.

I assume this would solve mine original problem. I wanted to add several column in 'data.table' which were in the form number + character.

M <- data.table('4PCS'=1:4,'5Y'=4:1,X5Y=2:5)
> M[,4PCS+5Y]
Error: unexpected symbol in "M[,4PCS"

新列应该是 4PSC 和 5Y 的总和.

The new column should be a sum of 4PSC and 5Y.

有没有办法在 data.table 中以不带引号的形式引用它们?如果这些列在 data.table 中引用了 data.frame 的引用逻辑":

Is there a way how to refer to them in data.table in no quoted form? If these columns are referred in data.table with the quoted "logic" of data.frame :

> M[,'5Y',with=FALSE]
     5Y
[1,]  4
[2,]  3
[3,]  2
[4,]  1

那么此类参考的功能将受到限制.添加不起作用，因为它在 data.frame 中不起作用:

then there will be a limitation in functionality of such reference. The addition would not work as it does not work in data.frame:

> M[,'4PCS'+'5Y',with=FALSE]  
Error in "4PCS" + "5Y" : non-numeric argument to binary operator

data.table 功能将允许对列进行操作.我想在新的 data.table 逻辑中找到一个解决方案，因此我可以使用它通过 列名引用 来转换列的能力.

The data.table functionality would allow to operate over the columns. I would like to find a solution in the new data.table logic hence I can use its ability to transform the columns by column name referencing.

问题是:
如何引用以数字开头的列名，以便 data.table 逻辑理解它是一个列名.

The question is:
How to quote the column name which start with number so that the data.table logic would understand that it is a column name.

推荐答案

我想，这就是你要找的，不确定.data.table 与 data.frame 不同.请查看 快速介绍，然后是常见问题解答(必要时还可参考参考手册).

I think, this is what you're looking for, not sure. data.table is different from data.frame. Please have a look at the quick introduction, and then the FAQ (and also the reference manual if necessary).

require(data.table)
dt <- data.table("4PCS" = 1:3, y=3:1)
#    4PCS y
# 1:    1 3
# 2:    2 2
# 3:    3 1

# access column 4PCS
dt[, "4PCS"]

# returns a data.table
#    4PCS
# 1:    1
# 2:    2
# 3:    3

# to access multiple columns by name
dt[, c("4PCS", "y")]

或者，如果您需要访问列并且 not 导致 data.table 而不是向量，那么您可以使用 $ 符号:

Alternatively, if you need to access the column and not result in a data.table, rather a vector, then you can access using the $ notation:

dt$`4PCS` # notice the ` because the variable begins with a number
# [1] 1 2 3

# alternatively, as mnel mentioned under comments:
dt[, `4PCS`] 
# [1] 1 2 3

或者，如果您知道列号，您可以使用 [[.]] 访问如下:

Or if you know the column number you can access using [[.]] as follows:

dt[[1]] # 4PCS is the first column here
# [1] 1 2 3

<小时>

谢谢@joran.我想你正在寻找这个:

Thanks @joran. I think you're looking for this:

dt[, `4PCS` + y]
# [1] 4 4 4

基本上问题是 4CPS 在 R 中不是一个有效的变量名(尝试 4CPS <- 1，你会得到相同的意外符号"错误).所以要引用它，我们必须使用反引号 (compare`4CPS` <- 1)

Fundamentally the issue is that 4CPS is not a valid variable name in R (try 4CPS <- 1, you'll get the same "Unexpected symbol" error). So to refer to it, we have to use backticks (compare`4CPS` <- 1)

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