#include <iostream>
using namespace std;
 
 
int main()
{
	const double GAL_CONVERSION = 3.78533;
	const double LIT_CONVERSION = .264;
	double gallons, litres;
	char ans;
	const char error_neg[] = "Unable to calculate negative volume, please enter a positive amount\n";
 
 
	cout << "\nWelcome to the Volume Converter!\n" << "Enter 0 to quit" <<endl;
 
	do
	{	
		cout <<"Please insert the number of gallons you would like to convert: ";
		cin >> gallons;
 
		litres = gallons * GAL_CONVERSION;
 
		if (gallons > 0)
		{
			cout << "You have " << litres << " litres " << "if you have " << gallons;
 
			if (gallons == 1)
			cout << " gallon\n";
 
			else 
			cout << " gallons\n";
		}
 
		if (gallons < 0)
 
			cerr << error_neg;
		else;
 
	} while (gallons != 0);
 
 
		cout << "Do you want to convert litres to gallons? (y/n): ";
		cin >> ans;
 
	if (ans == 'y')
 
	{	do
		{	
			cout << "please enter the number of liters you would like to convert: ";
			cin >> litres;
 
			gallons = litres * LIT_CONVERSION;
 
			if (litres > 0)
			{
				cout << "You have " << gallons << " gallons " << "if you have " << litres;
 
				if (litres == 1)
				cout << " litre\n";
 
				else
				cout << " litres\n";
			}
 
			if (litres < 0)
 
				cerr << error_neg;
 
 
 
		}while (litres != 0);
	}
 
	else;
 
	cout << "Good-bye\a" << endl;
 
 
	return 0;
}

#include <iostream>
#include <string>
#include <sstream>
using namespace std;
 
int main()
{
    int gallons;
    std::string input;
    cout << "Enter gallons\";
    cin >> input;
    bool ok  = true;
    for(int i = 0; i < input.length(); i++)
    {
       if( !isdigit(input[i]) )
       {
            cout << "Error\n";
            ok = false;
       }
   }
   if( ok == true )
   {
         stringstream str(input);
         str >> gallons;
   }
 

 int num = 0;
 cout<<"Number please : ";
 cin >> num;
 
 while(!cin)//if input fails
{
   cin.clear();
   while(cin.get() != '\n')
     continue;
  cout<<"\nInvalid Input\n";
  cout<<"Try again : ";
  cin >> num;
}