Giving wrong number when giving change using %

Question

I am creating a system for giving back change. When I run the program I get: (Lets say i gave $100)

You have given 100.0 dollars.
Your change is 14.670000000000002 dollars.
You will recieve:
0.6700000000000017 dollars
0.1700000000000017 quarters
0.0700000000000017 dimes
0.020000000000001697 nickels
1.6965595595053173E-15 pennies

code

import java.util.Scanner;
public class RunGivingChange {

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        double given;
        double cost = 85.33;
        double change;
        int dollars;
        int quarters;
        int dimes;
        int nickels;
        int pennies;

        Scanner input = new Scanner(System.in);

        System.out.println("Your total is " + cost + " dollars.");
        System.out.println("How much money are you giving?");
        given = input.nextDouble();
        if (given >= cost)
        {
            System.out.println("You have given " + given + " dollars.");
            change = given - cost;
            System.out.println("Your change is " + change + " dollars.");
            System.out.println("You will recieve:");

            //dollars
            change = change % 1;
            System.out.println(change + " dollars");

            //quarters
            change = change % 0.25;
            System.out.println(change + " quarters");

            //dimes
            change = change % 0.10;
            System.out.println(change + " dimes");

            //nickels
            change = change % 0.05;
            System.out.println(change + " nickels");

            //pennies
            change = change % 0.01;
            System.out.println(change + " pennies");
        }
        else if (given < cost)
        {
            System.out.println("Not enough money.");
        }
        else
        {
            System.out.println("Error.");
        }

    }

    }

Recommendation/request for your next question at least: be as specific as you can. `It's giving the wrong number` doesn't help much. At least we'd need to know what you consider to be wrong: is it the precision, the result of your calculations, something else? — Thomas, Sep 22 '15 at 12:18

score 3 · Answer 1 · answered Sep 22 '15 at 12:04

3

You're using double precision floating point values to represent money.

Don't do that.

Floating point numbers give you around 15 significant figures of accuracy, but with a phenomenal range (and computational performance) that makes them useful for scientific usage.

Used a fixed point type or BigDecimal for money values.

answered Sep 22 '15 at 12:04

Bathsheba

231,907
34
361
483

Yup, that's pragmatic, but don't forget currencies that have 1,000 "cents" to the "dollar", for example Tunisian Dinars. That can "eat" up quite a lot of your integer. You'd be OK with 64 bit but not 32. – Bathsheba Sep 22 '15 at 12:14
I woult use joda-money library instead of reinventing the wheel – Gaskoin Sep 22 '15 at 13:15

score 0 · Answer 2 · edited May 23 '17 at 11:44

Modulo (%) doesn't work as you expect it to do (besides the precision problems):

2.67 % 1 won't result in 2 but in 0.67.

(for some more information have a look here: How do I use modulus for float/double?)

You'd probably want to calc Math.floor( 2.67 / 1) etc. and subtract the rounded value from change.

Alternatively, if change is always positive (should probably be) you can just cast (and thus cut) to an integer:

int dollars = (int)(change / 1);
change -= dollars;

int quarters = (int)(change / 0.25);
change -= 0.25 * quarters;

...

To overcome the precion issues either use BigDecimal or convert all values to pennies, i.e. change = 1467 pennies, 1 dollar = 100 pennies etc.

score 0 · Answer 3 · answered Sep 22 '15 at 12:20

Instead of having the cost in dollars you could express it in the smallest division (pennies) :

double cost = 85.33;// dollars
you could express it as :
int cost = 8533: .. //pennies

Then you could use a / and % sequence of operations to compute each of the currency division. Ex:

int dollars = change / 100; // how many dollars fit in that pennies amount
change = change % 100; // how much pennies are left

//repeat for the other subdivisions

score 0 · Answer 4 · edited May 23 '17 at 11:51

If you are using doubles, make sure to round to decimal places.

change = (double) Math.round(change *100) / 100;

Second, mod (%) returns the remainder of the statement

14.67 % 1.0 = 0.67

What you want to do first is find how many times 1.0 will divide into 14.

dollars = Math.floor(change / 1);
change = change % 1.0;

This will round down to the nearest integer, and will give you how many times the 1.0 can go into the change.

The problem with using a double here is like what Bathsheba said. To fix this, you will have to round after every time change is modified.

This should give you the correct outputs.

import java.util.Scanner;

public class RunGivingChange{


    public static void main(String[] args) {


        double given;
        double cost = 85.33;
        double change;
        int dollars;
        int quarters;
        int dimes;
        int nickels;
        int pennies;

        Scanner input = new Scanner(System.in);

        System.out.println("Your total is " + cost + " dollars.");
        System.out.println("How much money are you giving?");
        given = input.nextDouble();
        if (given >= cost) {
            System.out.println("You have given " + given + " dollars.");
            change = given - cost;
            // round the change
            change = (double) Math.round(change *100) / 100;
            System.out.println("Your change is " + change + " dollars.");
            System.out.println("You will recieve:");

            //dollars     

            System.out.println(Math.floor(change / 1) + " dollars");
            change = change % 1;
            change = (double) Math.round(change *100) / 100;
            //quarters

            System.out.println(Math.floor(change / 0.25) + " quarters");
            change = change % 0.25;
            change = (double) Math.round(change *100) / 100;
            //dimes

            System.out.println(Math.floor(change / 0.1) + " dimes");
            change = change % 0.10;
            change = (double) Math.round(change *100) / 100;
            //nickels

            System.out.println(Math.floor(change / 0.05) + " nickels");
            change = change % 0.05;
            change = (double) Math.round(change *100) / 100;
            //pennies

            System.out.println(Math.floor(change / 0.01) + " pennies");

        } else if (given < cost) {
            System.out.println("Not enough money.");
        } else {
            System.out.println("Error.");
        }

    }

}

There are better ways to do this, next time you should search the site. There are multiple answers for this topic.

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4 Answers4