Arguments and classes for writing (generic) functions in R

Question

I want to make a small R package out of a few very simple functions. The literature I use is "Creating R packages: A tutorial", and "Writing R extensions". Although I tried, but I don't really understand the concept of generic functions and methods and how to handle the arguments within the different functions.

Here a small example of how my code looks like:

#Make generic function
f <- function(x,...) UseMethod("newmethod")

#Default method
f.default <- function(a,b=5,c=3,...){
    out <- a+b+c
    class(out) <- "fclass"
}

# Print method
print.f <- function(x,...){
    cat("Result:")
    print(x)
}

# Summary method
summary.f <- function(object,...){
    res <- object
    class(res) <- "fsummary"
    print(res)
}

# Plot method
plot.f <-function(x,p=0.3,...){}

I have a function called f with a default f.default. Actually my function needs several arguments (non of them is defined as x), so how do I have to make my generic function? The print method should simply print the output of f.default (in that simple case similar to the summary output). plot.f method uses the the output of f.default and one additional argument (obligatory). How can I write these functions correctly? The usual methods use arguments like "object" and "x"...but as I said I don't need any variable x in my functions... I am a little confused...maybe someone can help.

If there is someone out there how is willing to help me with this problem, I could also send the "real" R code (not only this fictive example).

Answer 1

You've got into a right mess here...

First, your constructor function probably shouldn't be a generic/method. Something like:

makefclass = function(a,b,c){
        l = list(a=a,b=b,c=c)
        class(l)="fclass"
        return(l)
      }

Then you can write print.fclass:

print.fclass=function(x,...){
     cat("I'm an fclass!")
     cat("my abc is ",x$a,x$b,x$c,"\n")
}

Then you do:

> f=makefclass(1,2,3)
> f
I'm an fclass!my abc is  1 2 2 

Hope that helps...

Answer 2

I've fixed your code and put in some comments on what I fixed...

UPDATE As @Spacedman pointed out, the "constructor" function should probably not be generic. But I'll keep it here so you see how a generic function is done.

#Make generic function
# This is the "constructor" function...
# ... UseMethod should have the name of the function!
f <- function(x,...) UseMethod("f")

#Default method
# ... The class name should be the same as the constructor
f.default <- function(a,b=5,c=3,...){
    out <- a+b+c
    class(out) <- "f"
    out # must return the object out, not the class name!
}

# Print method
# The "f" part of "print.f" must be the same as the class!
print.f <- function(x,...){
    cat("Result for f: ")
    print(unclass(x)) # Must unclass to avoid infinite recursion
    # NextMethod(x) # Alternative, but prints the class attribute...
}

# Summary method
# Should return a summary object (and not print it!)
# Need a unique class for it ("fsummary")
summary.f <- function(object,...){
    res <- object
    class(res) <- "fsummary"
    res
}

# Now need to print the summary too:
print.fsummary <- function(x, ...) {
    cat("f summary!\n")
    # Nice summary print goes here...
}

# Plot method
plot.f <-function(x,p=0.3,...){ cat("PLOTTING!\n") }

# Try it out:

x <- f(3)
x # print x

y <- summary(x) # 
y # print summary

plot(x)