The last topic that we're going to examine is how to define basic arithmetic operations on assignments. In particular, we'll examine how to perform arithmetic operations on two NumericGrade objects and how to add a set of values to a NumericGrade object. This can be done in the R environment by extending one of the sets of generic groups that is used to collect similar operations. The different groups include the Arith, Compare, Ops, Logic, Math, Math2, Summary, and Complex groups. The Ops group includes the Arith, Compare, and Logic operations, and we extend this group to include objects from the NumericGrades class.

We use the setMethod function for the Ops group to define operations involving NumericGrade objects. The resulting function obtains the grades and performs the requisite operation using the callGeneric command. The quirk of the system is that it must return a NumericGrade object, and the last step is to set the value of the grades for the first...