C# rounding of double not working

Question

I have this code to round some value. I am using Math.Round() for the same.

double h = 128.015999031067; double d = Math.Round(h, 3) * 1000;

result is 128015.99999999999 if I write it to console it gives me 128016. I want the value to be 128016

Am I missing any type conversions? Or there is some other way of doing this?

But it is working as you expected - https://dotnetfiddle.net/szBxpQ - the output is `128016` — Felipe Oriani, Jul 17 '18 at 11:38
Very carefully consider your numerics here, IEEE 754 `float` and `double` cannot accurately represent all numbers. Verify that these types will work correctly for the calculations you wish to make. — Mgetz, Jul 17 '18 at 11:38
See https://stackoverflow.com/questions/24859628/c-sharp-float-and-double-bug/24859646#24859646 — Magnus, Jul 17 '18 at 11:39
@FelipeOriani let me know if it prints Match https://dotnetfiddle.net/kQB4Xk — Shirish11, Jul 17 '18 at 12:02
You can consider using `decimal` https://dotnetfiddle.net/FCcepy — Felipe Oriani, Jul 17 '18 at 12:07
@FelipeOriani I know that's one solutions. Just trying to avoid type conversion changes in my code. Thanks — Shirish11, Jul 17 '18 at 12:12

score 8 · Answer 1 · answered Jul 17 '18 at 11:43

128.016 cannot be represented exactly in binary. There is no way that you will get 128016 by multiplying with 1000.

There are multiple ways around this:

Do nothing. It is already printed correctly. (Unless you want to perform further calculations.)
The "obvious" solution would be to simply round again.
The "simplest" solution would be to multiply first, then round.
The most "correct" solution would be, if you need exact values, you should use an exact type; there are many implementations of bignum, rational, decimal, and precise arithmetic available via NuGet, also consider using decimal.

I want to perform a value match of the rounded value which it fails. And I am rounding it again as u mentioned. I cannot round first as the values will be different. Its a general implementation which works for all other values except this one. — Shirish11, Jul 17 '18 at 12:06

score 0 · Answer 2 · answered Jul 17 '18 at 11:39

0

Have you tried using the round function without any other arguments?

double h = 128.015999031067; double d = Math.Round(h*1000);

answered Jul 17 '18 at 11:39

Ralph King

2 Answers2