Reimplementing our grading policy in Software

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412 Reimplementing our grading policy
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The grade function in 41/52 assumes that we already know the student's overall homework grade, and not just the individual homework assignments' grades How we obtain that grade is part of our policy: We used the average in 31/36 and the median in 322/47 Accordingly, we might wish to express this part of our grading policy in a function, along the same lines as we did in 41/52:
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// compute a student's overall grade from midterm and final exam grades // and vector of homework grades // this function does not copy its argument, because median does so for us double grade(double midterm, double final, const vector<double>& hw) { if (hwsize() == 0) throw domain_error("student has done no homework"); return grade(midterm, final, median(hw)); }
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This function has three points of particular interest The first point is the type, const vector<double>&, that we specify for the third argument This type is often called "reference to vector of const double" Saying that a name is a reference to an object says that the name is another name for the object So, for example, if we write
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vector<double> homework; vector<double>& hw = homework; // hw is a synonym for homework
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we are saying that hw is another name for homework From that point on, anything we do to hw is equivalent to doing the same thing to homework, and vice versa Adding a const, as in
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// chw is a read-only synonym for homework const vector<double>& chw = homework;
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still says that chw is another name for homework, but the const promises that we will not do anything to chw that might change its value Because a reference is another name for the original, there is no such thing as a reference to a reference Defining a reference to a reference has the same effect as defining a reference to the original object For example, if we write
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// hw1 and chw1 are synonyms for homework; chw1 is read-only vector<double>& hw1 = hw; const vector<double>& chw1 = chw;
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then hw1 is another name for homework, just as hw is, and chw1, like chw, is another name for homework that does not allow write access If we define a nonconst reference a reference that allows writing we cannot make it refer to a const object or reference, because doing so would require permission that the const denies Therefore, we cannot write
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vector<double>& hw2 = chw; // error: requests write access to chw
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because we promised not to modify chw Analogously, when we say that a parameter has type const vector<double>&, we are asking the implementation to give us direct access to the associated argument, without copying it, and also promising that we won't change the parameter's value (which would otherwise change the argument too) Because the parameter is a reference to const, we can call this function on behalf of both const and nonconst vectors Because the parameter is a reference, we avoid the overhead of copying the argument The second point of particular interest about the grade function is that like the one in 41/52, it is named grade even though it calls the other grade function The notion that we can have several functions with the same name is called overloading, and figures prominently in many C++ programs Even though we have two functions with the same name, there is no ambiguity, because whenever we call grade, we will supply an argument list, and the implementation will be able to tell from the type of the third argument which grade function we mean The third point is that we check whether homeworksize() is zero, even though we know that median will do so for us The reason is that if median discovers that we are asking for the median of an empty vector, it throws an exception that includes the message median of an empty vector This message is not directly useful to someone who is computing student grades Therefore, we throw our own exception, which we hope will give the user more of a clue as to what has gone wrong
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