Three kinds of function parameters in Software

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We would like to pause at this point for an important observation: We have defined three functions median, grade, and read_hw that operate on homework vectors Each of these functions treats the corresponding parameter in a fundamentally different way from the others, and each treatment has a purpose The median function ( 411/53) has a parameter of type vector<double> Therefore, calling that function causes the argument to be copied, even though that argument might be a huge vector Despite the inefficiency, vector<double> is the right parameter type for median, because this type ensures that taking the median of a vector doesn't change the vector The median function sorts its parameter If it did not copy its argument, then calling median(homework) would change the value of homework The grade function that takes a homework vector ( 412/54) has a parameter of type const vector<double>& In this type, the & asks the implementation not to copy the argument, and the const promises that the program will not change the parameter Such parameters are an important technique for making programs more efficient They are a good idea whenever the function will not change the parameter's value, and the parameter is of a type, such as vector or string, with values that might be time-consuming to copy It is usually not worth the bother to use const references for parameters of simple built-in types, such as int or double Such small objects are usually fast enough to copy that there's little, if any, overhead in passing them by value The read_hw function has a parameter of type vector<double>&, without the const Again, the & asks the implementation to bind the parameter directly to the argument, thus avoiding having to copy the argument But here, the reason to avoid the copy is that the function intends to change the argument's value Arguments that correspond to nonconst reference parameters must be lvalues that is, they must be nontemporary objects Arguments that are passed by value or bound to a const reference can be any value For example, suppose we have a function that returns an empty vector: vector<double> emptyvec() { vector<double> v; // no elements return v; }
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We could call this function, and use the result as an argument to our second grade function from 412/54: grade(midterm, final, emptyvec());
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When run, the grade function would throw an exception immediately, because its argument is empty However, calling grade this way would be syntactically legal When we call read_hw, both of its arguments must be lvalues, because both parameters are nonconst references If we give read_hw a vector that is not an lvalue read_hw(cin, emptyvec()); // error: emptyvec() is not an lvalue
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the compiler will complain, because the unnamed vector that we create in the call to emptyvec will disappear as soon as read_hw returns If we were allowed to make this call, the effect would be to store input in an object that we couldn't access!
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415 Using functions to calculate a student's grade
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This document is created with the unregistered version of CHM2PDF Pilot The whole point of writing these functions is to use them in solving problems For example, we can use them to reimplement our grading program from 322/46: // include directives and using-declarations for library facilities // code for median function from 411/53 // code for grade(double, double, double) function from 41/52 // code for grade(double, double, const vector<double>&) function from 412/54 // code for read_hw(istream&, vector<double>&) function from 413/57 int main() { // ask for and read the student's name cout << "Please enter your first name: "; string name; cin >> name; cout << "Hello, " << name << "!" << endl; // ask for and read the midterm and final grades cout << "Please enter your midterm and final exam grades: "; double midterm, final; cin >> midterm >> final; // ask for the homework grades cout << "Enter all your homework grades, " "followed by end-of-file: "; vector<double> homework; // read the homework grades read_hw(cin, homework); // compute and generate the final grade, if possible try { double final_grade = grade(midterm, final, homework); streamsize prec = coutprecision(); cout << "Your final grade is " << setprecision(3) << final_grade << setprecision(prec) << endl; } catch (domain_error) { cout << endl << "You must enter your grades " "Please try again" << endl; return 1; } return 0; }
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The changes from the earlier version are in how we read the homework grades, and in how we calculate and write the result After asking for our user's homework grades, we call our read_hw function to read the data The while statement inside read_hw repeatedly reads homework grades until we hit end-of-file or encounter a data value that is not valid as a double The most important new idea in this example is the try statement It tries to execute the statements in the { } that follow the try keyword If a domain_error exception occurs anywhere in these statements, then it stops executing them and continues with the other set of { } -enclosed statements These statements are part of a catch clause, which begins with the word catch, and indicates the type of exception it is catching If the statements between try and catch complete without throwing an exception, then the program skips the catch clause entirely and continues with the next statement, which is return 0; in this example Whenever we write a try statement, we must think carefully about side effects and when they occur We must assume that anything between try and catch might throw an exception
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This document is created with the unregistered version of CHM2PDF Pilot If it does so, then any computation that would have been executed after the exception is skipped What is important to realize is that a computation that might have followed an exception in time does not necessarily follow it in the program text For example, suppose that we had written the output block more succinctly as // this example doesn't work try { streamsize prec = coutprecision(); cout << "Your final grade is " << setprecision(3) << grade(midterm, final, homework) << setprecision(prec); }
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The problem with this rewrite is that although the implementation is required to execute the << operators from left to right, it is not required to evaluate the operands in any specific order In particular, it might call grade after it writes Your final grade is If grade throws an exception, then the output might contain that spurious phrase Moreover, the first call to setprecision might set the output stream's precision to 3 without giving the second call the opportunity to reset the precision to its previous value Alternatively, the implementation might call grade before writing any output; whether it does so depends entirely on the implementation This analysis explains why we separated the output block into two statements: The first statement ensures that the call to grade happens before any output is generated A good rule of thumb is to avoid more than one side effect in a single statement Throwing an exception is a side effect, so a statement that might throw an exception should not cause any other side effects, particularly including input and output Of course, we cannot run our main function as written We need the include directives and using-declarations for the library facilities that the program uses We also use the names read_hw and the grade function that takes a const vector<double>& third argument The definitions of these functions, in turn, use the median function and the grade function that takes three doubles In order to execute this program, we have to ensure that those functions are defined (in the proper order) before our main function Doing so yields an inconveniently large program Rather than write it out directly here, we'll see in 43/65 how we can partition such programs more succinctly into files Before we do so, let's look at better ways to structure our data
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