C# double subtraction

Question

I need to do some work with variables of the type double and in the calculations I've come across something I do not understand. I know floating point types are not exact but it has enough precision for what I need.

First I multiply a value of 200 with a multiplier of 1.1 and get 220.00000000000003 which is expected and close enough to 220 and then I subtract that from the expected value (220). The result should be 0 or very close to 0. Instead I get -2.8421709430404007E-14 which I cannot explain.

using System;

namespace Test
{
    class Program
    {
        static void Main(string[] args)
        {
            double u = 200;
            double v = 1.1;
            double x = u * v;
            double y = 220;
            double z = y - x;

            Console.WriteLine(u + " * " + v + " = " + x);

            Console.WriteLine(y + " - " + x + " = " + z);

            Console.ReadLine();
        }
    }
}

Any ideas?

`-2.8421709430404007E-14` is close to zero (i.e. move the decimal place 14 times to the *left*). — crashmstr, Oct 27 '15 at 19:32
-2.8421709430404007E-14 means `-2.84 * 10 ^ -14`. really close. more information about [Scientific notation](https://en.wikipedia.org/wiki/Scientific_notation) — M.kazem Akhgary, Oct 27 '15 at 19:33
In other words, the result is very close to (but not quite) `-0.00000000000003` — crashmstr, Oct 27 '15 at 19:34

score 2 · Accepted Answer · answered Oct 27 '15 at 19:38

That is a number very close to 0. It's scientific notation; the E-14 means 10^-14. In other words, shift the decimal point 14 places to the left.

There are libraries that make working with floating-point operations a little more intuitive; otherwise, you'll need to figure out and test against a tolerance in order to see whether you're "close enough."

score 0 · Answer 2 · answered Oct 28 '15 at 14:58

0

You should look into using the "decimal" type - your code looks like C#.net. This has high precision and is designed to work as you would expect (for financial calculation etc.).

Give it a try! :-)

answered Oct 28 '15 at 14:58

Th3Minstr3l

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C# double subtraction

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