enter not returning line?!

[Josh Henry]

ok so now the only thing I need to know how to do is that if the return is hit and no entry is made in the initial input line the cin will return as zero and run the while loop to produce cost = 0 tendered = 0 change = 0?! please help

#include <cmath>
#include <cstdlib>
#include <cctype>

// Placed all std in right #include
#include <iostream>
using std::cout;
using std::cin;
using std::endl;
using std::ios;

#include <iomanip>
using std::setprecision;
using std::setw;


int main()
{
// Beginning information on lab and programmer

  cout << endl;
  cout << "Lab: 2" << endl;
  cout << "Programmer: Joshua Henry" << endl;
  cout << endl;

// Declare variables for cost, pay and change

  double pur = 0;
  double tend = 0;
  int change;
  double balAmt = 0.0;
  const int FIFTY = 5000;
  const int TWENTY = 2000;
  const int TEN = 1000;
  const int FIVE = 500;
  const int ONE = 100;
  const int FIFTYCENT = 50;
  const int QUARTER = 25;
  const int DIME = 10;
  const int NICKEL = 5;
  const int PENNY = 1;

  
  int fiftyBill = 0;
  int twentyBill = 0;
  int tenBill = 0;
  int fiveBill = 0;
  int oneBill = 0;
  int fiftyCent = 0;
  int quarterCent = 0;
  int dimeCent = 0;
  int nickelCent = 0;
  int pennyCent = 0;

// Declare variables to compute what change given

// Ask the user to input both purchase and tendered amount

  cout << "Please enter the Purchase amount and amount tendered(seperated by a space): \n";
  cin >> pur >> tend;
  cout << endl;
 
  int cost = pur * 100;
  int pay = tend * 100;
 
// make operations to determine how many bills the change entitles
  change = pay - cost;
 

  while (change > 0)
  {
    if (change >= 5000)
    { 
      fiftyBill = change / FIFTY;
      change = change - (FIFTY * fiftyBill); 
    } 
      else if (change >= 2000)
      {
        twentyBill = change / TWENTY;
        change = change - (TWENTY * twentyBill);
      }
      else if (change >= 1000)
      {
        tenBill = change / TEN;
        change = change - (TEN * tenBill);
      }
      else if (change >= 500)
      { 
        fiveBill = change / FIVE;
   	change = change - (FIVE * fiveBill);
      }
      else if (change >= 100)
      {
	oneBill = change / ONE;
	change = change - (ONE * oneBill);
      }
      else if (change >= 50)
      {
	fiftyCent = change / FIFTYCENT;
 	change = change - (FIFTYCENT * fiftyCent);
      }
      else if (change >= 25)
      {
	quarterCent = change / QUARTER;
 	change = change - (QUARTER * quarterCent);
      }
      else if (change >= 10)
      { 
	dimeCent = change / DIME;
 	change = change - (DIME * dimeCent);
      }
      else if (change >= 5)
      { 
	nickelCent = change / NICKEL;
    	change = change - (NICKEL * nickelCent);
      }
      else if (change >= 1)
      {
 	pennyCent = change / PENNY;
	change = change - (PENNY * pennyCent);
      }
    
    }
 balAmt = tend - pur;

  
  cout.setf(ios::fixed|ios::showpoint);
  cout << setprecision(2);
  cout << endl;
  
  cout << "For a purchase of " << pur << " the amount tendered is " << tend << endl;
  cout << "Your change will be $" << balAmt << endl;

 if (fiftyBill != 0)
      cout << fiftyBill << " $50 bill" << endl;
if (twentyBill != 0)
      cout << twentyBill << " $20 bill" << endl;
if (tenBill != 0)
      cout << tenBill << " $10 bill" << endl;
if (fiveBill != 0)
      cout << fiveBill << " $5 bill" << endl;
if (oneBill != 0)
      cout << oneBill << " $1 bill" << endl;
if (fiftyCent != 0)
      cout << fiftyCent << " 50 cent coin" << endl;
if (quarterCent != 0)
      cout << quarterCent << " 25 cent coin" << endl;
if (dimeCent != 0)
      cout << dimeCent << " 10 cent coin" << endl;
if (nickelCent != 0)
      cout << nickelCent << " 5 cent coin" << endl;
if (pennyCent != 0)
      cout << pennyCent << " 1 cent coin" << endl;
      cout << endl;

}