How do you use double to get the expected result

Question

I was trying out subtracting numbers in java, and this gives me unexpected result

public class FloatWeird
{
    public static void main(String[] args)
    {
        double n = 1;

        for(int i = 0;i<10;i++)
        {
            System.out.println(n);
            n = n - 0.10;
        }

    }
}

Result

1.0
0.9
0.8
0.7000000000000001
0.6000000000000001
0.5000000000000001
0.40000000000000013
0.30000000000000016
0.20000000000000015
0.10000000000000014

I have gone through a few forums and understand that using the BigDecimal class is one solution. However, is there a way to correct it in a simpler way using double as above?

What do these numbers represent? Let's say they represent the amount of gin you need to use to make a cocktail. Does it really matter if you add a millionth of a billionth of liter more than needed? — JB Nizet, Aug 22 '15 at 09:05
IMHO This is not a duplicate as the OP specifically states they are aware of the alternative of using BigDecimal. — Peter Lawrey, Aug 22 '15 at 09:16

Peter Lawrey · Answer 1 · 2015-08-22T09:13:40.643

I suggest you use appropriate rounding.

System.out.printf("%.1f%n", n);

When ever you print a double you have to consider what the appropriate round is.

You can also round the result as you calculate which is what BigDecimal does.

n = n - 0.10;
n = Math.round(n * 10) / 10.0;

This will reduce cumulative error.

Another approach is to work in a different unit, e.g. instead of dollar you use cents. This means you can use an int or long and only convert to dollars for presentation.

long n = 100;
for(int i = 0; i < 10; i++) {
    System.out.println(n / 100.0);
    n = n - 10;
}

this prints http://ideone.com/Uf70jC

1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1

Note: even if you use BigDecimal, this still have to do this except the API can help you determine at what point you should do this.

How do you use double to get the expected result

1 Answers1