Rounding values and storing the rounded value in a variable

[bagerbeiger]

I am relatively new to programming (3-5 months) and have to write a program that prompts users for input (number of items ordered at a restaurant), performs some monetary calculations, and then requests monetary input/payment. I am using Dev-C++ by Bloodshed on Windows XP.

I am running into a problem with rounding the total expense to two decimal places.

EX: If you input 5 for numOfBurgers, numOfFries, and numOfSodas, the total to be paid comes to $17.861 and I need it to round down (not necessarily truncate) to 17.86 so that the payment portion always works properly.

I have read online to add 0.005 to my double, multiply it by 100, cast it to an int, cast it back to a double, and then divide by 100. I begin this process on line 47, and the payment portion of the program begins on line 55.

This methodology works, and the program runs properly every time, but I am not convinced that this is good practice, and can find no reference to situations like this in any of my textbooks or on the web. I would like to know what the best practice would be for tackling problems like this (rounding and truncating values stored in memory).

I have supplied my code below (using Dev-C++ by Bloodshed, Windows XP), thanks in advance for any help!

/*
  Name: FastFood.cpp
  Written by Calvin Kosterman
  Last modified on December 14th at 11:48pm
  Description: Program that receives a fast food order from a customer, 
               calculate the total expenses including GST, prompts the user for
               their payment, and calculates the customer's change accordingly.
*/
 
#include <iostream>
 
using namespace std;
 
int main()
 
{ // Begin int main()
 
   int        
      numOfBurgers,     // Number of burgers ordered
      numOfFries,       // Number of fries ordered
      numOfSodas,       // Number of sodas ordered
      totalBillToInt;   // Truncates totalBill
 
   const double
      burgerPrice = 1.49,      // Price for one burger
      friesPrice = 0.89,       // Price for one order of fries
      sodaPrice = 0.99,        // Price for one soda
      salesTax = 1.06;         // Sales tax charged on order 
 
   double
      totalBill,               // Total amount charged for order
      totalBillToDouble,       // Truncated value of totalBill
      amountTendered,          // Amount of money paid towards totalBill
      amountRemaining,         // Amount of money unpaid towards totalBill
      change;                  // Amount of money overpaid towards totalBill
 
 
         cout << "Enter number of burgers ordered\n";
         cin >> numOfBurgers;
         cout << "\n\nEnter number of fries ordered\n";
         cin >> numOfFries;
         cout << "\n\nEnter number of sodas ordered\n";
         cin >> numOfSodas;
         totalBill = (((numOfBurgers * burgerPrice) + (numOfFries * friesPrice) + (numOfSodas * sodaPrice)) * salesTax);
 
 
   totalBillToInt = ((totalBill + 0.005) * 100);       // Rounds totalBill
   totalBillToDouble = totalBillToInt;                 // to nearest penny
 
   totalBillToDouble = (totalBillToDouble / 100);      // Rounded value of 
                                                       // totalBill
 
   cout << "\n\nYour total comes to: $" << totalBillToDouble << endl << endl;
 
   cout << "\n\nEnter amount tendered:\n";
   cin >> amountTendered;
 
   amountRemaining = (totalBillToDouble - amountTendered);
 
   while (amountRemaining > 0.00)
 
   {   // Begin while (amountRemaining > 0.00) 
 
      cout << "You still owe: $" << amountRemaining << endl;
      cout << "\n\nEnter amount tendered:\n";
      cin >> amountTendered;
      amountRemaining = (amountRemaining - amountTendered);   
 
      if (amountRemaining < 0.00)
 
         {   // Begin if (amountRemaining < 0.00)
 
            change = (amountRemaining * -1.00);    
            cout << "\n\nYour change is $" << change << "\n\n\n***Please come again!***\n\n\n";
            system("PAUSE");
            return 0;
 
         }   // End if (amountRemaining < 0.00)
 
   }   // End while (amountRemaining > 0.00)
 
 
      if (amountRemaining == 0.00)
 
         {   // Begin if (amountRemaining == 0.00)
 
            cout << "\n\n\n***Please come again!***\n\n\n" ;
            system("PAUSE");
            return 0;
 
         }   // End if (amountRemaining == 0.00)
 
 
      if (amountRemaining < 0.00)
 
         {   //Begin if (amountRemaining < 0.00)
 
            change = (amountRemaining * -1.00);    
            cout << "\n\nYour change is $" << change << "\n\n\n***Please come again!***\n\n\n";
            system("PAUSE");
            return 0;
 
         }   // End if (amountRemaining < 0.00)
 
} // End int main()

[weaknessforcats]

First, you are using C++, so please stop casting. A C++ cast usually means a) you are calling relic C function that has a void* argiment or some such, or b) your design is screwed up. A cast always tells the compiler: Shut up and just do what I tell ya!!

Second, floating point cannot be used for money applications dues to the automatic rounding. In fact, in the EU there are laws prohibiting this practice. Floating point is intended for scientific work where extreme precision is not required. Beginners fall into this trap because of the handy decimal point.

Third, you are using integer operators on floating point. Read this:

Generally, you cannot use operators like ==, !=, >, <, <=, >=, etc wth floating point numbers.
Due to the automatic rounding, these operators may report a condition as true when the numbers
are only close in value.

Google for Floating Point Arithmetic or read IEEE 754 standard specification for details.

To compare two floating point numbers, you have to establish a sigma error tolerance.

if (fabs(i - j) < 0.000001) { ... // almost equal }
 

----------------------------------------------------------------------

Also, be aware of this:

float var = 1.1;
 

Symbolic constants like 1.1 are double. A double is larger than a float so your compiler is giving
you a warning about truncation and possible loss of data as the double is converted to a float.

You should create the constant as a float by coding:

float j = 1.1f;    /* or 1.1F */
 

Further, float is a type developed in the years of computers with small memories.
Unless you can write a specific reason on paper as to why you need a float,
I recommend you use double throughout.

Fourth, use integers in your program and keep the money in pennies. An int with a value of 1761 can be displayed as $17.61 or $1761, etc as determined by the display function.

[bagerbeiger]

Thank you weaknessforcats, that makes perfect sense now.