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
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