R: Create and Save a Method

setMethod {methods} | R Documentation

## Create and Save a Method

### Description

Create and save a formal method for a given function and list of classes.

### Usage

    setMethod(f, signature=character(), definition,
              where = topenv(parent.frame()),
              valueClass = NULL, sealed = FALSE)

    removeMethod(f, signature, where)

### Arguments

`f` | A generic function or the character-string name of the function.
`signature` | A match of formal argument names for `f` with the character-string names of corresponding classes. See the details below; however, if the signature is not trivial, you should use `method.skeleton` to generate a valid call to `setMethod`.
`definition` | A function definition, which will become the method called when the arguments in a call to `f` match the classes in `signature`, directly or through inheritance.
`where` | the environment in which to store the definition of the method. For `setMethod`, it is recommended to omit this argument and to include the call in source code that is evaluated at the top level; that is, either in an R session by something equivalent to a call to `source`, or as part of the R source code for a package. For `removeMethod`, the default is the location of the (first) instance of the method for this signature.
`valueClass` | Obsolete and unused, but see the same argument for `setGeneric`.
`sealed` | If `TRUE`, the method so defined cannot be redefined by another call to `setMethod` (although it can be removed and then re-assigned).

### Details

The call to `setMethod` stores the supplied method definition in the metadata table for this generic function in the environment, typically the global environment or the name space of a package. In the case of a package, the table object becomes part of the name space or environment of the package. When the package is loaded into a later session, the methods will be merged into the table of methods in the corresponding generic function object.

Generic functions are referenced by the combination of the function name and the package name; for example, the function `"show"` from the package `"methods"`. Metadata for methods is identified by the two strings; in particular, the generic function object itself has slots containing its name and its package name. The package name of a generic is set according to the package from which it originally comes; in particular, and frequently, the package where a non-generic version of the function originated. For example, generic functions for all the functions in package `base` will have `"base"` as the package name, although none of them is an S4 generic on that package.

Multiple packages can have methods for the same generic function; that is, for the same combination of generic function name and package name. Even though the methods are stored in separate tables in separate environments, loading the corresponding packages adds the methods to the table in the generic function itself, for the duration of the session.

The class names in the signature can be any formal class, including basic classes such as `"numeric"`, `"character"`, and `"matrix"`. Two additional special class names can appear: `"ANY"`, meaning that this argument can have any class at all; and `"missing"`, meaning that this argument must not appear in the call in order to match this signature. Don't confuse these two: if an argument isn't mentioned in a signature, it corresponds implicitly to class `"ANY"`, not to `"missing"`. See the example below. Old-style (‘S3’) classes can also be used, if you need compatibility with these, but you should definitely declare these classes by calling `setOldClass` if you want S3-style inheritance to work.

Method definitions can have default expressions for arguments, but a current limitation is that the generic function must have some default expression for the same argument in order for the method's defaults to be used. If so, and if the corresponding argument is missing in the call to the generic function, the default expression in the method is used. If the method definition has no default for the argument, then the expression supplied in the definition of the generic function itself is used, but note that this expression will be evaluated using the enclosing environment of the method, not of the generic function. Note also that specifying class `"missing"` in the signature does not require any default expressions, and method selection does not evaluate default expressions. All actual (non-missing) arguments in the signature of the generic function will be evaluated when a method is selected—when the call to `standardGeneric(f)` occurs.

It is possible to have some differences between the formal arguments to a method supplied to `setMethod` and those of the generic. Roughly, if the generic has ... as one of its arguments, then the method may have extra formal arguments, which will be matched from the arguments matching ... in the call to `f`. (What actually happens is that a local function is created inside the method, with the modified formal arguments, and the method is re-defined to call that local function.)

Method dispatch tries to match the class of the actual arguments in a call to the available methods collected for `f`. If there is a method defined for the exact same classes as in this call, that method is used. Otherwise, all possible signatures are considered corresponding to the actual classes or to superclasses of the actual classes (including `"ANY"`). The method having the least distance from the actual classes is chosen; if more than one method has minimal distance, one is chosen (the lexicographically first in terms of superclasses) but a warning is issued. All inherited methods chosen are stored in another table, so that the inheritance calculations only need to be done once per session per sequence of actual classes. See Methods for more details.

The function `removeMethod` removes the specified method from the metadata table in the corresponding environment. It's not a function that is used much, since one normally wants to redefine a method rather than leave no definition.

### Value

These functions exist for their side-effect, in setting or removing a method in the object defining methods for the specified generic.
The value returned by `removeMethod` is `TRUE` if a method was found to be removed.

### References

Chambers, John M. (2008) Software for Data Analysis: Programming with R Springer. (For the R version.)

Chambers, John M. (1998) Programming with Data Springer (For the original S4 version.)

### See Also

`method.skeleton`, which is the recommended way to generate a skeleton of the call to `setMethod`, with the correct formal arguments and other details.

Methods and the links there for a general discussion, `dotsMethods` for methods that dispatch on “...”, and `setGeneric` for generic functions.

### Examples

    require(graphics)
    ## methods for plotting track objects (see the example for setClass)
    ##
    ## First, with only one object as argument:
    setMethod("plot", signature(x="track", y="missing"),
      function(x,  y, ...) plot(slot(x, "x"), slot(x, "y"), ...)
    )
    ## Second, plot the data from the track on the y-axis against anything
    ## as the x data.
    setMethod("plot", signature(y = "track"),
     function(x, y, ...) plot(x, slot(y, "y"), ...)
    )
    ## and similarly with the track on the x-axis (using the short form of
    ## specification for signatures)
    setMethod("plot", "track",
     function(x, y, ...) plot(slot(x, "y"), y,  ...)
    )
    t1 <- new("track", x=1:20, y=(1:20)^2)
    tc1 <- new("trackCurve", t1)
    slot(tc1, "smooth") <- smooth.spline(slot(tc1, "x"), slot(tc1, "y"))$y #$
    plot(t1)
    plot(qnorm(ppoints(20)), t1)
    ## An example of inherited methods, and of conforming method arguments
    ## (note the dotCurve argument in the method, which will be pulled out
    ## of ... in the generic.
    setMethod("plot", c("trackCurve", "missing"),
    function(x, y, dotCurve = FALSE, ...) {
      plot(as(x, "track"))
      if(length(slot(x, "smooth") > 0))
        lines(slot(x, "x"), slot(x, "smooth"),
             lty = if(dotCurve) 2 else 1)
      }
    )
    ## the plot of tc1 alone has an added curve; other uses of tc1
    ## are treated as if it were a "track" object.
    plot(tc1, dotCurve = TRUE)
    plot(qnorm(ppoints(20)), tc1)

    ## defining methods for a special function.
    ## Although "[" and "length" are not ordinary functions
    ## methods can be defined for them.
    setMethod("[", "track",
      function(x, i, j, ..., drop) {
        x@x <- x@x[i]; x@y <- x@y[i]
        x
      })
    plot(t1[1:15])

    setMethod("length", "track", function(x)length(x@y))
    length(t1)

    ## methods can be defined for missing arguments as well
    setGeneric("summary") ## make the function into a generic

    ## A method for summary()
    ## The method definition can include the arguments, but
    ## if they're omitted, class "missing" is assumed.

    setMethod("summary", "missing", function() "<No Object>")

[Package methods version 2.8.1 Index]