CODEWITHSUNDEEP
rmethods
moodymudskipper
moodymudskipper

Reputation: 47300

How do I know which method will be called?

If I type methods(print) I get a long list of methods.

For an object of class data.frame, print.data.frame will be called.

It's not always so simple though:

hw <- "hello world"
class(hw) # [1] "character"

There is no print.character method. How do I know which method is called when executing print(hw)?

Upvotes: 2

Views: 367

Answers (2)

AkselA
AkselA

Reputation: 8836

Have you read Hadley's Advanced R and the chapter on objects? It might not give you the whole answer, but fundamentally what you're experiencing is the difference between method dispatch in C and regular S3 behaviour.

[ isn't really an R function, it's a C function and the decision of what method to use is done in C. That doesn't mean you can't create an S3 method for [ (or sum, +, [<- and other .Primitive functions), but when you do it's more like you're making a wrapper/preprocess for the C function, which R will dispatch, before the ultimate decision is made by the C function based on classes defined separate from your regular (and extensible) R classes.

Or at least that's how I've understood it.

Upvotes: 1

G. Grothendieck
G. Grothendieck

Reputation: 269471

Turn debugging on for print and then run your example:

> debug(print)
> print("hello")
debugging in: print("hello")
debug: UseMethod("print")
Browse[2]>  <---------------------------- press Enter to step forward
debugging in: print.default("hello")  <-- this is the method that gets called
debug: {
    noOpt <- missing(digits) && missing(quote) && missing(na.print) && 
        missing(print.gap) && missing(right) && missing(max) && 
        missing(useSource) && missing(...)
    .Internal(print.default(x, digits, quote, na.print, print.gap, 
        right, max, useSource, noOpt))
}

Upvotes: 4

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