这里是我要做的：

## Create an example of a data.table "dt" whose columns you want to index 
## using a character vector "xx"
library(data.table)
dt <- data.table(mtcars)
xx <- c("wt", "mpg")

## Construct a call object identical to that produced by quote(list("wt", "mpg"))
jj <- as.call(lapply(c("list", xx), as.symbol))

## Try it out
dt[1:5,eval(jj)]
#       wt  mpg
# 1: 2.620 21.0
# 2: 2.875 21.0
# 3: 2.320 22.8
# 4: 3.215 21.4
# 5: 3.440 18.7

计算语言，这样常常有助于看看结构你试图构造的对象。基于以下内容（一旦你知道 as.call（）和 as.symbol（）），所需的语言对象变成一块蛋糕：

When "computing on the language" like this, it's often helpful to have a look at the structure of the object you're trying to construct. Based on the following (and once you know about as.call() and as.symbol()), creating the desired language object becomes a piece of cake:

x <- quote(list(wt, mpg))

str(x)
#  language list(wt, mpg)

class(x)
# [1] "call"

str(as.list(x))
# List of 3
#  $ : symbol list
#  $ : symbol wt
#  $ : symbol mpg