为了说明程序员是如何开发算法,本博文以求全班平均成绩问题的两种具体形式为例,
算法详述:计数器控制的循环
本节考虑的问题陈述是:一个有10名学生的班级进行了一次测验,测验成绩(1~100中的一个整数)现在都已经出来了.请计算并打印本班学生的总成绩及班级的平均成绩.我们知道,班级的平均成绩等于总成绩除以学生的人数.
计数器控制的循环的伪代码算法
下面用代码列出要运行的动作,并指定这些动作运行的顺序.使用计数器控制的循环以一次一个的方式输入每个学生的成绩.这种方法用一个称为计数器的变量控制一组语句运行次数.在此,给出了一个已完全开发好的伪代码算法以及GradeBook类的一个版本,其中,GradeBook类在一个C++成员函数中实现了算法.
采用计数器控制的循环来解决求全班平均成绩问题的伪代码算法
采用计数器控制的循环来解决求全班平均成绩问题:GradeBook的头文件
// GradeBook.h // Definition of class GradeBook that determines a class average. // Member functions are defined in GradeBook.cpp #include <string> // program uses C++ standard string class using std::string; // GradeBook class definition class GradeBook { public: GradeBook( string ); // constructor initializes course name void setCourseName( string ); // function to set the course name string getCourseName(); // function to retrieve the course name void displayMessage(); // display a welcome message void determineClassAverage(); // averages grades entered by the user private: string courseName; // course name for this GradeBook }; // end class GradeBook
采用计数器控制的循环来解决求全班平均成绩问题:GradeBook的源代码文件
// GradeBook.cpp // Member-function definitions for class GradeBook that solves the // class average program with counter-controlled repetition. #include <iostream> using namespace std; #include "GradeBook.h" // include definition of class GradeBook // constructor initializes courseName with string supplied as argument GradeBook::GradeBook( string name ) { setCourseName( name ); // validate and store courseName } // end GradeBook constructor // function to set the course name; // ensures that the course name has at most 25 characters void GradeBook::setCourseName( string name ) { if ( name.length() <= 25 ) // if name has 25 or fewer characters courseName = name; // store the course name in the object else // if name is longer than 25 characters { // set courseName to first 25 characters of parameter name courseName = name.substr( 0, 25 ); // select first 25 characters cout << "Name \"" << name << "\" exceeds maximum length (25).\n" << "Limiting courseName to first 25 characters.\n" << endl; } // end if...else } // end function setCourseName // function to retrieve the course name string GradeBook::getCourseName() { return courseName; } // end function getCourseName // display a welcome message to the GradeBook user void GradeBook::displayMessage() { cout << "Welcome to the grade book for\n" << getCourseName() << "!\n" << endl; } // end function displayMessage // determine class average based on 10 grades entered by user void GradeBook::determineClassAverage() { int total; // sum of grades entered by user int gradeCounter; // number of the grade to be entered next int grade; // grade value entered by user int average; // average of grades // initialization phase total = 0; // initialize total gradeCounter = 1; // initialize loop counter // processing phase while ( gradeCounter <= 10 ) // loop 10 times { cout << "Enter grade: "; // prompt for input cin >> grade; // input next grade total = total + grade; // add grade to total gradeCounter = gradeCounter + 1; // increment counter by 1 } // end while // termination phase average = total / 10; // integer division yields integer result // display total and average of grades cout << "\nTotal of all 10 grades is " << total << endl; cout << "Class average is " << average << endl; } // end function determineClassAverage
增强GradeBook有效性确认
考虑一下对GradeBook类的增强工作.成员函数setCourseName将验证课程名称的合法性.它首先用一条if语句测试课程名称的的长度是否小于或者等于25个字符.如果是这样,接受此课程名称.
// Create GradeBook object and invoke its determineClassAverage function. #include "GradeBook.h" // include definition of class GradeBook int main() { // create GradeBook object myGradeBook and // pass course name to constructor GradeBook myGradeBook( "CS101 C++ Programming" ); myGradeBook.displayMessage(); // display welcome message myGradeBook.determineClassAverage(); // find average of 10 grades return 0; // indicate successful termination } // end main
算法详述:标记控制的循环
伪代码算法可以解决更一般化的求全班平均成绩的问题.如下所示.
采用标记控制的循环求全班平均成绩问题的伪代码算法
在GradeBook类中实现标记控制的循环
采用标记控制的循环求全班平均成绩问题:GradeBook头文件
// GradeBook.h // Definition of class GradeBook that determines a class average. // Member functions are defined in GradeBook.cpp #include <string> // program uses C++ standard string class using std::string; // GradeBook class definition class GradeBook { public: GradeBook( string ); // constructor initializes course name void setCourseName( string ); // function to set the course name string getCourseName(); // function to retrieve the course name void displayMessage(); // display a welcome message void determineClassAverage(); // averages grades entered by the user private: string courseName; // course name for this GradeBook }; // end class GradeBook
采用标记控制的循环求全班平均成绩问题:GradeBook源代码文件
// GradeBook.cpp Member-function definitions for class GradeBook that solves the // class average program with sentinel-controlled repetition. #include <iostream> using std::cout; using std::cin; using std::endl; using std::fixed; // ensures that decimal point is displayed #include <iomanip> // parameterized stream manipulators using std::setprecision; // sets numeric output precision // include definition of class GradeBook from GradeBook.h #include "GradeBook.h" // constructor initializes courseName with string supplied as argument GradeBook::GradeBook( string name ) { setCourseName( name ); // validate and store courseName } // end GradeBook constructor // function to set the course name; // ensures that the course name has at most 25 characters void GradeBook::setCourseName( string name ) { if ( name.length() <= 25 ) // if name has 25 or fewer characters courseName = name; // store the course name in the object else // if name is longer than 25 characters { // set courseName to first 25 characters of parameter name courseName = name.substr( 0, 25 ); // select first 25 characters cout << "Name \"" << name << "\" exceeds maximum length (25).\n" << "Limiting courseName to first 25 characters.\n" << endl; } // end if...else } // end function setCourseName // function to retrieve the course name string GradeBook::getCourseName() { return courseName; } // end function getCourseName // display a welcome message to the GradeBook user void GradeBook::displayMessage() { cout << "Welcome to the grade book for\n" << getCourseName() << "!\n" << endl; } // end function displayMessage // determine class average based on 10 grades entered by user void GradeBook::determineClassAverage() { int total; // sum of grades entered by user int gradeCounter; // number of grades entered int grade; // grade value double average; // number with decimal point for average // initialization phase total = 0; // initialize total gradeCounter = 0; // initialize loop counter // processing phase // prompt for input and read grade from user cout << "Enter grade or -1 to quit: "; cin >> grade; // input grade or sentinel value // loop until sentinel value read from user while ( grade != -1 ) // while grade is not -1 { total = total + grade; // add grade to total gradeCounter = gradeCounter + 1; // increment counter // prompt for input and read next grade from user cout << "Enter grade or -1 to quit: "; cin >> grade; // input grade or sentinel value } // end while // termination phase if ( gradeCounter != 0 ) // if user entered at least one grade... { // calculate average of all grades entered average = static_cast< double >( total ) / gradeCounter; // display total and average (with two digits of precision) cout << "\nTotal of all " << gradeCounter << " grades entered is " << total << endl; cout << "Class average is " << setprecision( 2 ) << fixed << average << endl; } // end if else // no grades were entered, so output appropriate message cout << "No grades were entered" << endl; } // end function determineClassAverage
采用标记控制的循环求全班平均成绩问题:创建GradeBook类的一个对象
// Create GradeBook object and invoke its determineClassAverage function. // include definition of class GradeBook from GradeBook.h #include "GradeBook.h" int main() { // create GradeBook object myGradeBook and // pass course name to constructor GradeBook myGradeBook( "CS101 C++ Programming" ); myGradeBook.displayMessage(); // display welcome message myGradeBook.determineClassAverage(); // find average of 10 grades return 0; // indicate successful termination } // end main
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