Using R for Introductory Statistics : John Verzani

Appendix E 382if (!is.String(x)) {x = as.character(x)class(x) = "String"}return(x)}is.String=function(x) return(class(x) == "String")(We write these functions as though they are “sourced” in from a file. The good way toorganize this would be to include all these new functions in a single file, as they are in afile in the UsingR package.)Now when a generic function is called on an instance of this class, the function for theString class, if present, is used. For example, the String class should have some basicfunctions, such as length (), summary (), and print ()length.String = function(x,…) return(nchar(x))summary.String = function(x,…)return(table(strsplit(x,"")))print.String = function(x,…) cat(x,"\n")At this point we can see if it works as expected. For example:> bart=string("I will not skateboard in the halls")> bartI will not skateboard in the halls> length(bart)[1] 34> summary(bart)I a b d e h i k l n o r s t w6 1 3 1 1 2 2 2 1 4 2 2 1 2 3 1The summary () function splits the string into characters and then makes a table. Bymodifying the summary. string () function other summaries could be given.It would be nice to be able to access the letters in a String object as we do the elementsin a vector. First, let’s define a function slice() and then use this to define access via "[".To do so, slice () is made a “generic” function in order for method dispatch to work, andthen defined as desired:slice = function(x,index) UseMethod("slice")slice.String = function(x,index){tmp = unlist(strsplit(x,""))[index]return(string(paste(tmp,sep="",collapse="")))}To make slice () generic, we used UseMethod(). The definition of slice () uses strsplit ()to split the string into characters.To make the vector notation work on the String class, we overload the square-bracketmeaning by defining a function as follows: