c++中在跳出循环时cin.get()递增的问题

Question

所以我试图写一个气泵程序。它正在编译和工作，但我唯一的问题是当我打印出最终的加仑量并充电时。当我按q然后回车退出循环并得到答案时，它又增加了一次。看看displaycalc()函数。

#include <iostream>
using namespace std;

class GasPump
{
    private:
        double gallons;
        double charge;
        double gasInTank ;
        double costPerGallonReg;
        double costPerGallonPlus;
        double costPerGallonSup;
        double costPerGallonChoice;

    public:
        void regularSetting();
        void setPricePerGallon(double,double,double);
        double getPricePerGallon();
        void displayCalc();
};
//first function
void GasPump::regularSetting()
{
  gallons = 0;
  charge = 0;
  gasInTank = 10.0;

  cout<<"Price "<<charge<<endl;
  cout<<"Gallons "<<gallons;
}
void GasPump::setPricePerGallon(double reg, double plus, double super)
{   
    cout<<"nnPlease choose type of gas you want to Purchasen";
    cout<<"nRegular: "<<reg<<" Plus: "<<plus<<" Premium: "<<super;
    //setting prices
    costPerGallonReg = reg;
    costPerGallonPlus = plus;
    costPerGallonSup = super;
}
double GasPump::getPricePerGallon()
{
    char choice;
    cout <<"nEnter r/R for Regular, p/P for Plus, or s/S for Super.n";
    cin >>choice;
    if ((choice == 'R' || choice == 'r'))
    {
        return  costPerGallonChoice = costPerGallonReg ; 
    }
    else if ((choice == 'P'|| choice == 'p'))
    {
        return  costPerGallonChoice = costPerGallonPlus ;
    }
    else if ((choice == 'S'|| choice == 's'))
    {
        return  costPerGallonChoice = costPerGallonSup ;
    }
    else
    {
        cout<<"Invalid";
    }
}
void GasPump::displayCalc()
{
     cout.setf(ios::fixed);
     cout.setf(ios::showpoint);
     cout.precision(2);
    char choice;
    system("cls");
    cout<<"Press Enter to start filling and q to quitn";
    cout<<"Price "<<charge<<endl;
    cout<<"Gallons "<<gallons<<endl;
    while(gasInTank > 0)
   { 
    choice = cin.get();
     if((choice =='Q'||choice =='q'))
     break;
     system("cls");
     cout<<"$"<<charge<<" ";
     cout<<"Gallons "<<gallons<<endl;
     charge += costPerGallonChoice * 0.1;
     gallons += 0.1;
     gasInTank -= 0.1;
  }
    cout<<"nThank you for your purchasen";
    cout<<"Gallons: "<<gallons<<" $"<<charge;       
}




int main()
{
    //naming class
    GasPump pump;
    cout<<"Gas pump Programnn";
    pump.regularSetting();
    pump.setPricePerGallon(1.69,1.79,1.89);
    pump.getPricePerGallon();
    pump.displayCalc();

    getchar();getchar();
    return 0;
}

Answer 1

你的代码几乎是正确的，有2个问题:刷新cin流并在while循环中移动cout，代码如下:

 cout<<"$"<<charge<<" ";
 cout<<"Gallons "<<gallons<<endl;

放在while循环的末尾，比如

cin.ignore(1000, 'n');
cin.get();
cout<<"$"<<charge<<" ";
cout<<"Gallons "<<gallons<<endl; // if you really want to display the 0 
while(gasInTank > 0)
{ 
     choice = cin.get();
     if((choice =='Q'||choice =='q'))
         break;
     system("cls");
     // Move these 2 lines at the end
     // cout<<"$"<<charge<<" ";
     // cout<<"Gallons "<<gallons<<endl;
     charge += costPerGallonChoice * 0.1;
     gallons += 0.1;
     gasInTank -= 0.1;
     // moved here, now it's ok
     cout<<"$"<<charge<<" ";
     cout<<"Gallons "<<gallons<<endl;
}

否则在递增之前显示。基本上，当你按下第一个ENTER键时，你已经把汽油放进了油箱，变量增加了，但在你的原始代码中，你是在增加之前显示数量的。