Context

I want to use non-standard evaluation with dollar sign in R.

I want to customize a function with two parameters. data is the input data frame, var is the name of the variable in the input data frame. The return value is the value corresponding to the variable name.

I can do this with fun1.

library(dplyr)

df = data.frame(a = 1:3)

fun1 <- function(data, var){
  data %>% pull({{var}})
}

> fun1(data = df, var = a)
[1] 1 2 3

Question

Is there any way to perform the function of fun1 with non-standard evaluation and dollar signs($).

My solution is as follows (fun2) but it reports an error.

fun2 <- function(data, var){    # Can't create fun2, it will report an error.
  data${{var}}
}

fun2(data = df, var = a)

CodePudding user response：

We may use [[ to extract after converting the unquoted to string

fun2 <- function(data, var){  
     var <- deparse(substitute(var))
     data[[var]]
}

-testing

> fun2(data = df, var = a)
[1] 1 2 3

It is better to use [[ instead of $. In case, we want to only use $, then paste the names and eval (not recommended)

fun2 <- function(data, var) {
  argslist <- as.list(match.call()[-1])
  eval(parse(text = sprintf('%s$%s', 
    deparse(argslist$data), deparse(argslist$var))))
}
> fun2(df, a)
[1] 1 2 3