Test if an argument of a function is set or not in R

Question

I have a function f that takes two parameters (p1 and p2):

If for the parameter p2 no value was passed to the function, the value of p1^2 should be used instead. But how can I find out within the function, if a value is given or not. The problem is that the variable p2 is not initialized if there was no value. Thus I can't test for p2 being NULL.

f <- function(p1, p2) {     if(is.null(p2)) {         p2=p1^2     }     p1-p2 } 

Is it somehow possible to check if a value for p2 was passed to the function or not? (I could not find an isset() - function or similar things.)

Answer 1

You use the function missing() for that.

f <- function(p1, p2) {     if(missing(p2)) {         p2=p1^2     }     p1-p2 } 

Alternatively, you can set the value of p2 to NULL by default. I sometimes prefer that solution, as it allows for passing arguments to nested functions.

f <- function(p1, p2=NULL) {     if(is.null(p2)) {         p2=p1^2     }     p1-p2 }  f.wrapper <-function(p1,p2=NULL){     p1 <- 2*p1     f(p1,p2) } > f.wrapper(1) [1] -2 > f.wrapper(1,3) [1] -1 

EDIT: you could do this technically with missing() as well, but then you would have to include a missing() statement in f.wrapper as well.