CODEWITHSUNDEEP
.netmathfloating-point
user3306496
user3306496

Reputation: 29

Wrong floating numbers calculation in .NET

This situation really confused me: I have one .NET application doing some floating number calculation. It seems having problem to do the division, Math.Pow, and Math.Exp. For example:

double _E1 = 20616579.5;
double sub = 19000;
double total = 19623;
double percent = sub / total;                 //0.96825152635574341
double _result1 = Math.Pow(_E1, percent);     //12078177.0
double _result2 = Math.Exp(percent * Math.Log(_E1)); //12078184.730266357

all three results, percent, _result1, and _result2 are incorrect (you can use calculator to verify).

I have another .NET program running the same code, on the same machine, gives the correct results:

By just looking at the result for percent, the precision only goes to 7 decimal digits. The Double usually goes to 16 decimal digits.

I have another even simpler example as follows: enter image description hereenter image description here

_outcome equals to some ridiculous number. but it shows correct result when I put cursor on top of "*".

Please help, drove me crazy in the last few days.

UPDATE: the problem solved. DirectX was the culprit. see: Can floating-point precision be thread-dependent?

Upvotes: 1

Views: 241

Answers (2)

D Stanley
D Stanley

Reputation: 152491

Your exact code doesn't compile (you have percent declared twice). I suspect that there's a conversion somewhere else that you haven't posted (perhaps a conversion to float somewhere that will reduce precision to 6-7 digits).

When I run your code (minus the extra declaration of percent) in LinqPad I get the correct result:

void Main()
{
    double _E1 = 20616579.5;
    double sub = 19000;
    double total = 19623;
    double percent = sub / total;                 
    double _result1 = Math.Pow(_E1, percent);     
    double _result2 = Math.Exp(percent * Math.Log(_E1));  

    percent.Dump();     // 0.968251541558375
    _result1.Dump();    // 12078180.3702605
    _result2.Dump();    // 12078180.3702605
}

Upvotes: 1

ohlmar
ohlmar

Reputation: 956

Double only has the precision of 15-16 digits. I think this is where you get your error.

Try using Decimal (http://msdn.microsoft.com/en-us/library/364x0z75.aspx) instead. Can handle 28-29 digits.

Upvotes: 0

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