S3 Methods for Logical Operations

Question

Let's say that I have an S3 object:

obj <- list()
obj$A <- "This"
obj$B <- "That"
class(obj) <- "myclass"

How would I go about writing a method for myclass such that it referenced obj$B (for example) when I called it to create a logical vector

For example, I would like this behaviour:

obj == "That" # TRUE 
obj == "This" # FALSE

I'd like obj == "That" to return TRUE, but how do I tell == which part of the object to test?

How do I find documentation for this?

score 7 · Accepted Answer · answered Jan 15 '16 at 01:21

You should consider writing an Ops.myclss method. It gives you the ability to handle all group methods in a single function.

See ?base::Ops, and methods(Ops) for example implementations. There's also a bit of information in Section 5.6 - Group Methods of R Language Definition.

Here's an example Ops.myclass that is a slightly modified version of Ops.Date:

Ops.myclass <-
function (e1, e2) 
{
    if (nargs() == 1) 
        stop(gettextf("unary %s not defined for \"myclass\" objects", 
            .Generic), domain = NA)
    boolean <- switch(.Generic, `<` = , `>` = , `==` = , `!=` = , 
        `<=` = , `>=` = TRUE, FALSE)
    if (!boolean) 
        stop(gettextf("%s not defined for \"myclass\" objects", 
            .Generic), domain = NA)
    if (inherits(e1, "myclass"))
        e1 <- e1$B
    if (inherits(e2, "myclass"))
        e2 <- e2$B
    NextMethod(.Generic)
}

Note that comparison will work whether or not e1 or e2 are myclass objects. And it will also work for any classes that extend myclass but do not define their own Ops method (though the error message would be confusing).

Interesting, so if I'm reading this correctly, it doesn't just cover the `==` scenario but rather all potential logical operators? — Brandon Bertelsen, Jan 15 '16 at 01:47
@BrandonBertelsen: Yes, and some math operators, as listed in the *Details* section of `?base::Ops`. — Joshua Ulrich, Jan 15 '16 at 02:28

score 4 · Answer 2 · answered Jan 15 '16 at 00:01

4

What about :

`==.myclass` <- function(e1, e2){ e1$B == e2 }

> obj=="That"
[1] TRUE
> obj=="This"
[1] FALSE

answered Jan 15 '16 at 00:01

HubertL

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Where did you find documentation for `e1` and `e2`? – Brandon Bertelsen Jan 15 '16 at 00:20
[There](http://stackoverflow.com/questions/8022979/operator-overloading-and-class-definition-in-r-use-a-different-base-field-corpu) but it's pretty outdated – HubertL Jan 15 '16 at 00:21
1

HaHaHa. The syntax of base R might be "outdated" to some but it is relatively stable. – IRTFM Jan 15 '16 at 03:28

S3 Methods for Logical Operations

2 Answers2