程序设计基石与实践之标记控制和计数器控制的循环

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



[b]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
[/b]

// 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

[b]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[/b]

// 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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