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 中的列名采用数字+字符的形式，例如： 4PCS ， 5Y 等，如何将其称为 j 在 x [i，j] 中，以便将其解释为未引用的列名。

我认为这可以解决我原来的问题。我想在 data.table中添加几列，其形式为数字+字符。

  M<-data.table（'4PCS'= 1：4，'5Y'= 4：1，X5Y = 2：5）
> M [，4PCS + 5Y] 
错误： M [，4PCS中的意外符号
  

新列的总和应为 4PSC 和 5Y 。

有没有办法在 data.table 中以无引号的形式引用它们？如果这些列在 data.table 中用 data.frame 引用的逻辑进行引用：

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

然后，这种引用的功能将受到限制。添加项不起作用，因为它在 data.frame 中不起作用：

 > M [，'4PCS'+'5Y'，with = FALSE] 
 4PCS + 5Y中的错误：二进制运算符
   
 
 
  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，因此我可以使用它的功能通过列名引用来转换列。 
 
 
 
问题是：
 
如何引用以数字开头的列名，以便data.table逻辑会理解这是一个列名。
解决方案
我认为，这是您要查找的，不确定。  data.table 与 data.frame 是不同。请查看 快速简介 ，然后 常见问题解答 （如有必要，还提供参考手册）。
 需要（data.table）
 dt<-data.table（ 4PCS = 1：3，y = 3：1）
＃4PCS y 
＃1：1 3 
＃2：2 2 
＃3：3 1 
 
＃访问列4PCS 
 dt [， 4PCS] 
 
＃返回数据.table 
＃4PCS 
＃1：1 
＃2：2 
＃3：3 
 
＃通过名称$ b $访问多个列b dt [，c（ 4PCS， y）] 
  
或者，如果需要访问该列，而 not 生成 data.table 而不是向量，则可以使用 $ 表示法：
  dt $`4 PCS`＃注意`，因为变量以数字
＃[1] 1 2 3 
 
＃＃替代，如注释中提到的mnel：
 dt [，`4PCS `] 
＃[1] 1 2 3 
  
或者如果您知道列号，可以使用 [[。]] 进行访问，如下所示：
  dt [ [1]＃4PCS是此处的第一列
＃[1] 1 2 3 
  
 
 
 
 
 
 
 编辑： 
 
 
 
感谢@joran。我认为您正在寻找这样的东西：
  dt [，`4PCS` + y] 
＃[1] 4 4 4 
  
根本上，问题在于 4CPS 在R中不是有效的变量名（尝试 4CPS<-1 ，您将收到相同的意外符号错误）。因此要引用它，我们必须使用反引号（compare `4CPS`<-1 ）
 
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. 

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

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
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
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.   

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.
解决方案
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")]
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
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




Edit:

Thanks @joran. I think you're looking for this:
dt[, `4PCS` + y]
# [1] 4 4 4
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)

                        
这篇关于如何在data.table中引用以数字开头的列名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！