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I have written a coin change generator program. The user enters in a subtotal and the program shows the number of coins that will be dispensed based on the subtotal. When I run it, the program outputs the information correctly. However, there is an error showing at the balance when it gets to the pennies. Instead of showing the balance as just '0.0000', the output shows the balance as '-0.0000'. I don't understand why this occurs.

PS. I know you can account for absolute values using the math library, however, for the purposes of this assignment I cannot use it.

 #include <stdio.h>
    
    int main()
    {
    double total_subtotal;
    printf("Enter subtotal: ");
    scanf("%lf", &total_subtotal); //user enters subtotal

    int Toonies = ((total_subtotal * 100) >= 200) ? (int)((total_subtotal * 100) / 200) : 0;
    int change_toonie = (int)(total_subtotal * 100) % 200;
    double toonie_balance = total_subtotal - (double)(Toonies * 2); //calculating remaining balance after toonies dispensed
    
    int Loonies = change_toonie >= 100 ? (int)change_toonie / 100 : 0;
    int change_loonie = change_toonie % 100;
    double loonie_balance = toonie_balance - (double)Loonies; //calculating remaining balance after loonies dispensed
    
    int Quarters = change_loonie >= 25 ? (int)change_loonie / 25 : 0;
    int change_quarter = change_loonie % 25;
    double quarter_balance = loonie_balance - (double)Quarters * 0.25; //calculating remaining balance after quarters dispensed
    
    int Dimes = change_quarter >= 10 ? (int)change_quarter / 10 : 0;
    int change_dime = change_quarter % 10;
    double dime_balance = quarter_balance - (double)Dimes * 0.10; //calculating remaining balance after dimes dispensed
    
    int Nickels = change_dime >= 5 ? (int)change_dime / 5 : 0;
    int change_nickel = change_dime % 5;
    double nickel_balance = dime_balance - (double)Nickels * 0.05; //calculating remaining balance after nickels dispensed
    
    int Pennies = change_nickel >= 1 ? (int)change_nickel / 1 : 0;
    double penny_balance = nickel_balance - Pennies * 0.01; //calculating remaining balance after pennies dispensed
    
    printf("Sales EXCLUDING tax\n");
    printf("Coin Qty Balance\n");
    printf("%22.4lf \n", total_subtotal);
    printf("Toonies %3d %9.4lf\n", Toonies, toonie_balance);
    printf("Loonies %3d %9.4lf\n", Loonies, loonie_balance);
    printf("Quarters %3d %9.4lf\n", Quarters, quarter_balance);
    printf("Dimes %3d %9.4lf\n", Dimes, dime_balance);
    printf("Nickels %3d %9.4lf\n", Nickels, nickel_balance);
    printf("Pennies %3d %9.4lf\n", Pennies, penny_balance); //where the error occurs
    return 0;
    }

This is what the output looks like. The quantity is the number on the left and the remaining balance is the number on the right. I still have to format it correctly so please excuse the formatting.

 //user enters 323.51 as subtotal
    Sales EXCLUDING tax
    Coin Qty Balance
    -------- --- ---------
                  323.5100
    Toonies 161    1.5100
    Loonies   1    0.5100
    Quarters   2    0.0100
    Dimes   0    0.0100
    Nickels   0    0.0100
    Pennies   1   -0.0000
jps
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  • Probably has something to do with your precision and how the difference is actually negative number, but it can't print that many decimal numbers cause you set the precision to 4 decimal places. I'd reckon you should recalculate that manually, and be careful of truncating any values. – Asphodel Jan 26 '22 at 19:29
  • Related: [Does float have a negative zero? (-0f)](https://stackoverflow.com/questions/5095968/does-float-have-a-negative-zero-0f) – Weather Vane Jan 26 '22 at 19:35
  • Don't use floats for exact arithmetic, use integers. – stark Jan 26 '22 at 19:37
  • 1
    This is compiler-dependent: [on ideone, your code works the way you expect](https://ideone.com/pVUovc). This has to do with the tiny arithmetic error that gets accumulated during the subtractions. The best approach here is to get the float, multiply it by 100 right away, make it an int, and treat it as the number of pennies. – Sergey Kalinichenko Jan 26 '22 at 19:38
  • The questions should be reopened, because although we shouldn't use float for money, that's not what OP is finding here. He's finding that printf correctly prints -0.00000000something as -0.0000 if you chop off past the 4th digit. That's an interesting quirk, distinct from the behavior of comparison operators and the (correct) observation that money is not floating point. – Topological Sort Jan 26 '22 at 20:15

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