[R] Extract the names of the arguments in an "expression"

[Javier López-de-Lacalle]

Hi everybody:

I need to get the names of the arguments in an object of class "expression".
I've got the following expression:

> x <- expression(rho * cos(omega))

Is there any function or procedure that returns the names of the arguments
(in the example: "rho" and "omega")?

I tried a rough approach implemented in the function expr.args() shown
below. As the function eval() needs to get access to those arguments, a
possible approach is as follows: 1) apply eval() to the expression "x"
within an empty environment; 2) get the variable names from the character
string containing the error message that will be returned: 

"Error in eval(expr, envir, enclos) : object 'rho' not found";

3) assign a value to the first identified variable, "rho", and apply eval()
again until the expression is evaluated and no error returned.

There are some pitfalls in this approach, expr.args():

	i) it is a recursive procedure (I guess there must be a more
efficient approach); 

	ii) it does not work if some of the arguments, for instance 'rho',
exist in the workspace. Despite a new environment is created to evaluate the
expression, objects are also searched in the parent environment. The search
should somehow stick to the new environment (called 'tmpe' in expr.args()); 

	iii) it does not work if the name of an argument coincides with the
name of a function (for instance 'gamma').

Is there any function to do this task? If not, I would appreciate some
guidance to improve the function expr.args().

> expr.args <- function(x)
{
  cond <- is.expression(x)
  if (cond) {
    tmpe <- new.env()
  } else return()

  while (cond)
  {
    ref <- try(eval(x, envir = tmpe), silent = TRUE)    
    if (cond <- (class(ref) == "try-error"))
    {
      if (length(grep("not found", ref[1])) > 0)
      {
        aux <- substr(ref, regexpr("object ", ref) + 8, regexpr(" not
found", ref) - 2)
        assign(as.character(aux), 1, envir = tmpe)
      } else stop("expression could no be evaluated but a missing variable
was not identified.")
    }
  }

  ls(envir = tmpe)
}

Many thanks.

javi