Question

I need to have some float divisions that must be accurate like double version of them. I can change divided value – it represents a mapping and I can offset it – to correct eventual floating point errors.

To correct the errors, I use following code:

do 
{
    float fValue = float(x) / 1024.f;
    double oldFValue = fValue;
    double dValue = double(x) / 1024.0;
    if(oldFValue != dValue)
    {
        x += 1;
    }
    else
    {
        break;
    }
}while(1);

With this code, for

x = 11 

I have in debugger (Visual Studio 2010):

fValue = 0.010742188
oldFValue = 0.010742187500000000

Can you please explain why double value is different from float value? Is this a debugger problem or a floating point conversion problem? I’m asking this because:

if(oldFValue != dValue)

is never true, even though it should be. Should I compare the float value with the double value in some other way? I need the result of float division to be exactly same as the double division.

Answer 1

You have to read (and understand) What Every Computer Scientist Should Know About Floating-Point Arithmetic.