在R中使用变量在data.table中传递列名 [英] pass column name in data.table using variable in R

问题描述

可能重复：

可变选择/分配给data.table中的字段

在下面的示例中，我创建了一个数据表，其列名为'x'和'v'

In following example, I am creating a data table having column name ‘x’ and ‘v’

library('data.table')
DT = data.table(x=c("b","b","b","a","a"),v=rnorm(5)) DT$
DT$v  DT$x

'x'：

DT[,x]
[1] "b" "b" "b" "a" "a"

但是如果我想通过一个变量访问， t工作

But if I want to access by passing through a variable, it doesn’t work

 temp="x"
 DT[,temp]
[1] "x"

将有多个列，我必须为其中几个选择值。这些列名将通过R模块提供。

There would be multiple columns and I will have to select values for only couple of them. These column names I will be provide by passing through a R module.

没关系，我得到了

我得到它..
它应该是

I got it.. it shoud be

DT[,get(temp)]

推荐答案

使用quote（）和eval（）函数将变量传递给j。当您这样做时，不需要对列名称加双引号，因为quote（） - 编译的字符串将在DT []中进行计算。

Use the quote() and eval() functions to pass a variable to j. You don't need double-quotes on the column names when you do it this way, because the quote()-ed string will be evaluated inside the DT[]

temp <- quote(x)
DT[,eval(temp)]
# [1] "b" "b" "b" "a" "a"

使用单个列名称，结果是一个向量。如果你想要一个data.table结果或多个列，使用列表形式

With a single column name, the result is a vector. If you want a data.table result, or several columns, use list form

temp <- quote(list(x,v))
DT[,eval(temp)]
#   x           v
# 1: b  1.52566586
# 2: b  0.66057253
# 3: b -1.29654641
# 4: a -1.71998260
# 5: a  0.03159933

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