Error when trying to reference a variable/column inside a function

In R, I am trying find the maximum value of a variable in a dataset inside a function but I can’t figure out the correct way to reference the variable.

df <- data.frame(a = 1:3, b = 2:4)

I’ve written the function as

f <- function(var) {
  m <- max(df${{var}})
  if (m == 3) {
    print("It's 3.")
  }
}

with the intention to call it like so: f(a)

However, when trying the define the function I get the following error:

Error: unexpected '{' in:
  "f <- function(var) {
  m <- max(df${"

I appreciate any help with this.

>Solution :

Apparently you are trying to use tidy evaluation syntax, in particular “curly-curly”.

Tidy evaluation only works with specific functions which explicitly support it. To find out whether a function supports tidy evaluation, consult its documentation. But, in particular, no base-R function or operator supports it, including $.

And while you could redefine $, the syntax rules of the R language prohibit $ from supporting tidy evaluation even then, because the right-hand operator of $ must always be a bare name or a quoted string — nothing else is allowed.

Put differently, you cannot use $ with variables or parameters. Instead, you can use [[…]] subsetting. However, that also does not work with the curly-curly syntax. You need to deparse the argument name manually:

f <- function(var) {
  var_name <- as.character(substitute(var))
  m <- max(df[[var_name]])
  if (m == 3) {
    print("It's 3.")
  }
}