Question

给定一个字符串向量，我想创建一个没有引号的表达式。

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

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

我在删除引号时遇到了一些困难

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))")

<小时/> 这适用于data.table列选择，如果相关。
我正在寻找的应该能够适应data.table，如下所示：

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

Answer 1

这是我要做的：

## 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

当"computing on the language"像这样时，看一下你想要构建的对象的结构通常会有所帮助。基于以下内容（一旦您了解as.call()和as.symbol()），创建所需的语言对象就变得轻而易举：

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

Answer 2

我倾向于使用plyr包中的as.quoted

 outputString <- sprintf('list(%s)', paste(input, collapse = ', ')) 


 library(plyr)
  output <- as.quoted(outputString)[[1]]

  mydt[, eval(output)]
   string1 string2
1:       A       a
2:       B       b
3:       C       c

但是，如果只是列选择，则可以传递字符串并使用with = FALSE

mydt[, input, with = FALSE]
   string1 string2
1:       A       a
2:       B       b
3:       C       c

Answer 3

我发现这些答案有用但不完整，因为在表达式中使用变量和多行。要从字符串创建带引号的表达式，使用变量和多行来使用quote（），atop（）和subsititute（）：

  # Prepare variables
  samp_skewness = round(skewness(dv),2)
  samp_kurtosis = round(kurtosis(dv),2)
  samp_var = round(var(dv))
  samp_summ <- summary(dv)
  num_samples = length(dv)

  # Prepare quotes containing math annotations
  q1 = quote(paste(mu,"="))
  q2 = quote(paste(sigma,"="))
  q3 = quote(paste(gamma[1],"="))
  q4 = quote(paste(gamma[2],"="))

# Use subsitition to construct the expression, passing in the variables and quoted expressions
  title = substitute(atop(paste("Top Title, # samples: ", ns), 
            paste(e1,v1,", ",e2,v2,", ",e3,v3,", ",e4,v4)),
            list(ns=num_samples,v1=round(samp_summ['Mean']),v2=samp_var,
            v3=samp_skewness,v4=samp_kurtosis,e1=q1,e2=q2,e3=q3,e4=q4))

在ggplot中：...

labs(title = title) +

...

如何从字符串创建带引号的表达式

3 个答案: