C++ 开关结构（最后一次运行两次）答案

【问题标题】：C++ Switch Structure (Running Last Case Twice)C++ 开关结构（最后一次运行两次）
【发布时间】：2013-10-01 13:43:12
【问题描述】：

#include <iostream>
#include <iomanip>
#include <fstream>

using namespace std;

//Named constants residential customers
const double RES_BILL_PROC_FEES = 4.50;
const double RES_BASIC_SERV_COST = 20.50;
const double RES_COST_PREM_CHANNEL = 7.50;

//Named constants business customers
const double BUS_BILL_PROC_FEES = 15.00;
const double BUS_BASIC_SERV_COST = 75.00;
const double BUS_BASIC_CONN_COST = 5.00;
const double BUS_COST_PREM_CHANNEL = 50.00;

int main()
{
   //Variable declarations
    int accountNumber;
    char customerType;
    int numOfPremChannels;
    int numOfBasicServConn;
    double amountDue;

    //declaring file streams
    ifstream inFile;
    ofstream outFile;

    //open the file streams
    inFile.open("input.txt");
    outFile.open("output.txt");

    //check wheather file exists or not
    if (inFile.fail())
    {
        cout << "The input file cannot be found! ";
        return 1;
    }

    //output console message
    cout << "\n==========================================\n\n";
    cout << "     PROGRAM TO COMPUTE THE CABLE BILL\n";
    cout << "\n==========================================\n\n";
    //output outfile message
    outFile << "\n==========================================\n";
    outFile << "              Customer Details";
    outFile << "\n==========================================\n";

    //loop until end of file
    while (!inFile.eof())
    {
    cout << "The account number is: ";
    inFile >> accountNumber;
    cout << accountNumber<<"\n";

    cout << "The customer type: "
         << "R or r (Residential), "
         << "B or b (Business):  ";
    inFile >> customerType;
    cout << customerType << endl;

    //switch to residential or business customer type
    switch (customerType)
        {
        case 'r':
        case 'R':
            cout << "Enter the number"
                 << " of premium channels: ";
            cin >> numOfPremChannels;
            cout << "\n\n";

            amountDue = RES_BILL_PROC_FEES
                       + RES_BASIC_SERV_COST
                       + numOfPremChannels *
                         RES_COST_PREM_CHANNEL;
            //write to output file
            outFile << setw(25) << left << "\nCustomer Account Number: "
                    << accountNumber << endl;
            outFile << setw(25) << left << "Amount Due: "
                    << fixed << setprecision(2) << "$" << amountDue << endl;

            break;

        case 'b':
        case 'B':
            cout << "Enter the number of basic "
                 << "service connections: ";
            cin >> numOfBasicServConn;


            cout << "Enter the number"
                 << " of premium channels: ";
            cin >> numOfPremChannels;
            cout << "\n\n";

            if (numOfBasicServConn <= 10)
                amountDue = BUS_BILL_PROC_FEES
                            + BUS_BASIC_SERV_COST
                            + numOfPremChannels *
                              BUS_COST_PREM_CHANNEL;
            else
                amountDue = BUS_BILL_PROC_FEES
                            + BUS_BASIC_SERV_COST
                            + (numOfBasicServConn - 10) *
                               BUS_BASIC_CONN_COST
                            + numOfPremChannels *
                              BUS_COST_PREM_CHANNEL;

            //write to output file
            outFile << setw(25) << left << "\nCustomer Account Number: "
                    << accountNumber << endl;
            outFile << setw(25) << left << "Amount Due: "
                    << fixed << setprecision(2) << "$" << amountDue << endl;

            break;

        default:
            //write to output file
            outFile << "\nThe account number " << accountNumber << " has an invalid customer type." << endl;

            break;
        } //end switch

    } //end eof while loop

    //close input file
    inFile.close();

    //close output file
    outFile.close();

    return 0;
}

谁能向我解释为什么最后一个案例总是运行/打印两次？这是我们必须为课堂编写的基本程序，我似乎不明白为什么它不起作用。我的 .eof() while 循环有问题吗？我尝试过使用 do-while 循环，但得到了相同的结果。我还尝试通过不包含默认情况来更改输入文件，但它仍然运行最后一组两次。

**对于基本连接数和通道数，输入任意有效整数即可这是我的输入文件：

157744232 b
253453534 r
335345435 R
445345345 B
545345345 t

【问题讨论】：

C++ 对于这样的问题是相当严厉的。为什么不使用 Python、Ruby 甚至 NodeJS？
@tadman 如果他正在学习 C++ 课程，我认为他不会通过提交 Python 答案获得好成绩。
.eof() loop not working的可能重复
@Barmar 啊，我没有注意到这个要求。

标签： c++ while-loop switch-statement

【解决方案1】：

当您在文件上编写循环时，您应该使用如下内容：

while( inFile >> somevar ) {
   ...
}

或者这个：

while( true ) {
   inFile >> somevar1;
   if( !inFile ) break;
   ...
}

这是常见的错误。

【讨论】：

【解决方案2】：

这不是因为switch 结构，而是因为eof 调用：该函数仅在尝试读取下一项失败后才开始返回true。

将循环改为无限循环（for (;;) 或while(true)，无所谓），读取数据后对eof 进行检查。如果您收到true，请使用break 结束循环。否则，请继续您的switch 声明。

你的循环结构应该是这样的：

for (;;) {
    cout << "The account number is: ";
    if (!(inFile >> accountNumber))
        break;
    cout << accountNumber<<"\n";
    cout << "The customer type: "
         << "R or r (Residential), "
         << "B or b (Business):  ";
    if (!(inFile >> customerType))
        break;
    cout << customerType << endl;
    switch (customerType) {
        ... // The rest of your code
    }
}

【讨论】：