Hiding a result of a function in R

Question

I created a function that has four output arguments, for example:

myfuction<-function(...){     
    #Inside the function I created four results A, B, C, and D.    
    A = ...    
    B = ...    
    C = ...    
    D = ...     
    z<-list(MacKinnon=A,regression=B,proof=C, res=D)    
    return(z)
}

The result D corresponds to a vector of numbers that respresents the residuals of a regression.

My question is How can I hide this result without removing it? That is, I want that when I run the function, the results A, B, and C appear, but not the result D.

And if I want to access the result D, I have to do something like this:

X <-myfuction (...)
X$res

to be able to observe the residual.

Answer 1

I would just use an S3 class. Basically, just tag the object z with a particular class

myfunction <- function(){     
  #Inside the function I created four results A, B, C, and D.    
  A = 10;B = 20;C = 30; D = 40     
  z = list(MacKinnon=A, regression=B, proof=C, res=D)    
  class(z) = "my_fun" # Tagging here
  return(z)
}

Create an S3 print function for my_fun

print.my_fun = function(x, ...) print(x[1:3])

Then

R> x = myfunction()
R> x
$MacKinnon
[1] 10

$regression
[1] 20

$proof
[1] 30

But

R> x$res
[1] 40

gives you want you want.

---

A couple of comments/pointers.

• Typically when you assign the class, you would use something like

  class(z) = c("my_fun", class(z))
  

  however, since we just created z in the line above, this isn't needed.

• Currently the print method strips away any additional classes (in the example, there is only one class, so it's not a problem). If we wanted to maintain multiple class, we would use

  print.my_fun = function(x, ...) {
    x = structure(x[1:3], class = class(x)) 
    NextMethod("print")
  }
  

  The first line of the function subsets x, but maintains all other classes. The second line, then passes x to the next print.class_name function.