如何从字符串创建带引号的表达式 [英] how to create a quoted expression from strings

问题描述

给定一个字符串向量，我想创建一个没有引号的表达式。

Given a vector of strings, I would like to create an expression without the quotation marks.

# eg, I would like to go from 
c("string1", "string2")

# to...  (notice the lack of '"' marks)
quote(list(string1, string2))

我遇到一些困难删除引号

I am encountering some difficulty dropping the quotation marks

input <- c("string1", "string2")
output <- paste0("quote(list(", paste(input, collapse=","), "))")

# not quite what I am looking for.     
as.expression(output)
expression("quote(list(string1,string2))")

---

这是用于data.table列选择，

我要查找的内容应该能够适合data.table如下：

---

This is for use in data.table column selection, in case relevant.
What I am looking for should be able to fit into data.table as follows:

library(data.table)
mydt <- data.table(id=1:3, string1=LETTERS[1:3], string2=letters[1:3])

result <- ????? # some.function.of(input)
> mydt[ , eval( result )]
   string1 string2
1:       A       a
2:       B       b
3:       C       c

推荐答案

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

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