[Mono-list] Mono and .Net Floating Point Inconsistencies

[Kornél Pál]

Hi,

This seems to me a float32 rounding problem because when using float64 I 
get the same results.

Code:

using System;

class Testing
{
     public static void Main()
     {
         float f1 = 0;
         f1 += -0.7779751f;

	Console.Write("0x");
	foreach (byte b in BitConverter.GetBytes(f1))
		Console.Write (b.ToString("x2"));
	Console.WriteLine();
         Console.WriteLine(f1.ToString("R"));

         f1 += -1f * -1.42317927f;

	Console.Write("0x");
	foreach (byte b in BitConverter.GetBytes(f1))
		Console.Write (b.ToString("x2"));
	Console.WriteLine();
         Console.WriteLine(f1.ToString("R"));

         f1 += -1.30905056f * 0.241778925f;

	Console.Write("0x");
	foreach (byte b in BitConverter.GetBytes(f1))
		Console.Write (b.ToString("x2"));
	Console.WriteLine();
         Console.WriteLine(f1.ToString("R"));

         f1 = (2.0F / (1.0F + (float)Math.Exp(-4.9F * f1))) - 1.0F;

	Console.Write("0x");
	foreach (byte b in BitConverter.GetBytes(f1))
		Console.Write (b.ToString("x2"));
	Console.WriteLine();
         Console.WriteLine(f1.ToString("R"));
     }
}

The last value is different indeed (I only thested on Windows):
MS:
0xa9c02a3f
0,667002261

Mono:
0xa8c02a3f
0,6670022

Hex value is different for me as well.

Kornél

Robert Jordan wrote:
> ddambro wrote:
>> Hi,
>>
>> I was fairly sure the code had executed.  As I mentioned, different flags in
>> the C code caused errors in the program.  Just to be sure though, I threw in
>> some fprintf statements to stderr into setFloats() and they printed
>> properly, so the code was definitely executed.
>>
>> Thankfully I have been able to come up with some code that shows at least
>> one example of the inconsistencies I've been talking about.  Here it is:
> 
> It's just a string representation mismatch, as this code reveals the
> same bits:
> 
>          Console.WriteLine(f1.ToString("R"));
> 
> 	foreach (byte b in BitConverter.GetBytes(f1))
> 		Console.WriteLine (b);
> 
> Robert
> 
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