Using R for Introductory Statistics : John Verzani

Appendix E 385})We need to use the argument x in length() and object in summary() to match their formalarguments (signature). The signature of a function can be found from its help page (e.g.,?summary), or with the function args() (e.g., args (summary)).New methods can be created as well. For example, to create a slice () method and the"[" syntax, we first create a generic function slice (), adapt it to the String class, and thenlink "[" to this.setGeneric("slice", function(object,index)standardGeneric("slice"))This sets up slice () as a generic function for which method dispatch will apply. Thedefinition of the function sets up the signature of the slice () function, namely an objectand an index. The definition of slice() for the String class issetMethod("slice","String",function(object,index) {tmp =paste(unlist(strsplit(object@string,"")[index],sep="",collapse="")return(string(tmp))})To define the "[" method, we again use setMethod(). It does not need to be made generic,but the signature must match.setMethod("[","String".function(x,i,j,…,drop)slice(x,i))There is a long list of formal arguments. The "[" method works for vectors, data frames,arrays, and now strings, so it needs to have lots of flexibility in how it is called. We havelittle flexibility in how a new method is defined, though: at the minimum, we mustinclude all the arguments given.We can try this out:> pets=string("Snowball, Santa’s little helper")> pets[−1]nowball, Santa’s little helper> pets[length(pets)]rThe OOP packageFor those used to programming in an OOP language, the S3 and S4 methods might becalled “object oriented,” but they don’t look object oriented. Usually, the syntax forcalling a method is object-separator-methodname, such as $object->print () in PERL orobject .print () in Ruby, C++, and Java. The separators -> and . are not well suited for R,