when passing df$var to a function, is it possible to get the name of 'var'?

Question

I'm writing a function where I'd like to be able to pass in variables from a data frame as atomic vectors, like df$var (eg, mtcars$mpg ).

To keep the example very simple, say the function just returns data.frame(table(df$var)) :

foo.function <- function(var) {
  data.frame(table(var))
}

head(foo.function(mtcars$mpg))
#>    var Freq
#> 1 10.4    2
#> 2 13.3    1
#> 3 14.3    1
#> 4 14.7    1
#> 5   15    1
#> 6 15.2    2

Notice that the name of the tabulated variable in the returned table is the internal name of the passed object ( var ) rather than it's "original" name, which was mpg . Is it possible to retrieve mpg (just the name) from within the function (without changing or adding arguments)? I was inclined to say no, since R is just receiving a vector of values, but I suspect R may have this capacity based on what it can do with NSE .

Answer 1

We can use deparse/substitute to extract the column name

foo.function <- function(var) {
   print(sub(".*\\$", "", deparse(substitute(var))))
   data.frame(table(var))
  }

head(foo.function(mtcars$mpg), 4)
#[1] "mpg"
#   var Freq
#1 10.4    2
#2 13.3    1
#3 14.3    1
#4 14.7    1

---

If we need to change the column name

foo.function <- function(var) {
  nm1 <- sub(".*\\$", "", deparse(substitute(var)))
  out <- data.frame(table(var))
  names(out)[1] <- nm1
  out
 }

head(foo.function(mtcars$mpg), 4)
#  mpg Freq
#1 10.4    2
#2 13.3    1
#3 14.3    1
#4 14.7    1

As @RonakShah noted in the comments, it is better to pass column names and data as separate arguments. If the limitation of the function is to pass only a single argument and it always have to be with $ , then the above function would be able to retrieve the column name