Hello, I am writing a program about gas pump.
Here is my code

class GasPump{
public:
	GasPump();// loads all pumps w/ 50 gallons, price is 2.00, 2.25, 2.50
	GasPump(int l, int m = 100, int h = 100);
	//price is 2.00, 2.25, 2.50
	GasPump(double pl, double pm, double ph);
	// loads all pumps w/ 50 gallons
	
	void setHighGal(int gallons);
	void setMedGal(int gallons);
	void setLow(int gallons);

	void setHighPrice(double price);
	void setMedPrice(double price);
	void setLowPrice(double price);

	int getHigh();
	int getMed();
	int getLow();

	double getPriceHigh();
	double getPriceMed();
	double getPriceLow();
	
	bool emptyHigh();
	bool emptyMed();
	bool emptyLow();

	double salePriceHigh(int gal);
	double salePriceMed(int gal);
	double salePriceLow(int gal);

	double earnedHigh();
	double earnedMed();
	double earnedLow();

	void display(ostream& out) const;

private:
	int highOctane;
	int medOctane;
	int lowOctane;
	double pricehigh;
	double pricemed;
	double pricelow;
	double totalHigh;
	double totalMed;
	double totalLow;
};
GasPump::GasPump(int l, int m, int h){

    emptyHigh();
	emptyMed();
	emptyLow();    
}
	int main()
	{
        int ql, qm, qh, choice;		
	    double plow, pmed, phigh;
		cout << "Welcome to Chevron" << endl;
		cout << "Please enter the quantities of "
		     << "each grade(Regular, Plus, and Premium)" << endl;
        cin  >> ql >> qm >> qh;


        cout << "Now enter the price of each grade"
             << " (Regular, Plus, and Premium)" << endl;
        cin >> plow >> pmed >> phigh;
        cout << "Regular: " << ql << " gallons" << endl;
        cout << "Cost per gallon: $" << plow << endl << endl;;
        cout << "Plus: " << qm << " gallons" << endl;
        cout << "Cost per gallon: $" << pmed << endl << endl;
        cout << "Premium: " << qh << " gallons" << endl;
        cout << "Cost per gallon: $" << phigh << endl << endl;       	
		GasPump pump(ql, qm, qh);
		pump.setLowPrice(plow);
		pump.setMedPrice(pmed);
		pump.setHighPrice(phigh);
		
  do{
			cout << "Which gas type would you want like to buy?\n";
			cout << "\n" << "\n" << "1)Regular\n" << "2)Plus\n"
				 << "3)Premium\n" << "4)Exit\n";  
			cin >> choice;
			switch(choice){
                int gal, gal2;
				case 1: 
                    cout << "How many gallons would you like to buy? " << endl;
                    cin >> gal;
                    pump.salePriceLow(gal);
                    pump.display(cout);
                    break;
				case 2:
                    cout << "How many gallons would you like to buy? " << endl;
                    cin >> gal;
                    pump.salePriceMed(gal);
                    pump.display(cout);
                    break;
				case 3:
                    cout << "How many gallons would you like to buy? " << endl;
                    cin >> gal;
                    pump.salePriceHigh(gal2);
                    pump.display(cout); 
					break;
				default:
					break;
			}

		}while (choice != 4);
		system("PAUSE");
		return 0;
	}

void GasPump::setHighGal(int gallons){
// sets the quantity of gas in the indicated tank to parameter value
//resets total cash for that type of gas to 0     
	highOctane = gallons;
	totalHigh - 0;
}
void GasPump::setMedGal(int gallons){
// sets the quantity of gas in the indicated tank to parameter value
//resets total cash for that type of gas to 0	
	medOctane = gallons;
	totalMed = 0;
}
void GasPump::setLow(int gallons){	
// sets the quantity of gas in the indicated tank to parameter value
//resets total cash for that type of gas to 0
	lowOctane = gallons;
	totalLow = 0;
}
void GasPump::setHighPrice(double price){
//sets the price of gas in the indicated tank to parameter value
	pricehigh = price;
}
void GasPump::setMedPrice(double price){
//sets the price of gas in the indicated tank to parameter value	
	pricemed = price;
}
void GasPump::setLowPrice(double price){
//sets the price of gas in the indicated tank to parameter value	
	pricelow = price;

}
int GasPump::getHigh(){
//returns  # gallons of high octane
	return highOctane;
}
int GasPump::getMed(){
//returns  # gallons of medium octane    
	return medOctane;
}
int GasPump::getLow(){
//returns  # gallons of low octane	
	return lowOctane;
}

double GasPump::getPriceHigh(){
//returns price for high octane
	return pricehigh;
}
double GasPump::getPriceMed(){
//returns price for medium octane
	return pricemed;
}
double GasPump::getPriceLow(){
//returns price for low octane
	return pricelow;
}

double GasPump::salePriceHigh(int gal){
//purchase quantity of high octane, cost of this purchase is returned                       
    highOctane -= gal;
    return totalHigh =(gal*pricehigh);
}
double GasPump::salePriceMed(int gal){
//purchase quantity of meduim octane, cost of this purchase is returned
    medOctane -= gal;
    return totalMed = (gal*pricemed);
}

double GasPump::salePriceLow(int gal){
//purchase quantity of low octane, cost of this purchase is returned
    lowOctane =- gal;
	return totalLow =(gal*pricelow);
}
bool GasPump::emptyHigh(){
//is high octane empty?
	if (highOctane == 0)
		return true;
	else
		return false;
}
bool GasPump::emptyMed(){
//is medium octane empty?
	if (medOctane == 0)
		return true;
	else
		return false;
}
bool GasPump::emptyLow(){
//is low octane empty?	
    if (lowOctane == 0)
		return true;
	else
		return false;

}

double GasPump::earnedHigh(){
// returns total earned for high octane
	return totalHigh;	
}
double GasPump::earnedMed(){
// returns total earned for meduim octane	
    return totalMed;
}
double GasPump::earnedLow(){
// returns total earned for low octane	
	return totalLow;
}

void GasPump::display(ostream& out) const{
     out << "Total HIgh: " << totalHigh << endl;

}

Ok, in the switch statement, I am having some trouble calling the salePriceLow, salePriceMed, salePriceHigh function after I enter 40 gallons. The gal for the salePriceMed should automatically initialize 40.
I'll explain clearly what I want to do
First the user enters number of gallons per grade.
lets say I enter 50, 50, 50
then I enter the price $1, $2, $3
then it should ask the user to pick which grade I want to buy, and then the user ask how many gallons I buy? So I say 40. and it should output
You purchased 40 gallons and output the total cost.
-Buy a quantity of low grade gasoline, displaying the cost of the gasoline. If the low grade tank is empty, display a message to that effect. If the required quantity of gasoline is not available, dispense what is left, and output a message to that effect.
I was take care of med grade, and high grade after you help me solve this problem

I know its long, any help would be appreciated.
Thanks

First the moan - as this is your 9th post you should know about code tags. Please use them, you'll get more help.
Now some problems:
GasPump::salePriceHigh() is being called with an uninitialised variable (gal2).
GasPump::display() is only outputting the value for totalHigh so obviously won't be correct for low and medium. Think about using a display method for each grade.
That's all I can see immediately, if you still have problems, try and be more specific about the error.

Sorry, but how specific do you want to be? My english is not good, and I'm not sure I understand.

Thanks

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