Re: [Mingw-users] MinGW and Floating Point

[Ben R. <Ben...@sc...>]

There is something very wrong somewhere because if you are performing

x / 640.0 *  9e-6

For 0.0 <= x <= 640.0

Your results should be way less than 1.  For x = 640 the 
result is 9e-6 and for anything else it is less.

I tried

    int value;
    
    for(value = 100; value < 120; value ++)
    {
        std::cout << value << ": " << (double(value)/640.0*9e-6) <<
std::endl;
    }

and got

100: 1.40625e-006
101: 1.42031e-006
102: 1.43438e-006

Using

>g++ --version
g++ (GCC) 4.4.0
Copyright (C) 2009 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is
NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR
PURPOSE.

>From the mingw project.

What you are getting is more than a truncation error, the 
result is not even in the right ball park I would want to 
see further evidence before I started questioning the compiler.

-----Original Message-----
From: Richard C. Wagner [mailto:drsavage@q.com] 
Sent: 04 June 2010 01:38
To: MinGW Users
Subject: [Mingw-users] MinGW and Floating Point

Hi All:

Has anyone here made much use of MinGW with scientific or floating point
calculations?  I've written a Mandelbrot generator, and I'm seeing
truncation errors at values far above where I expect them.
Specifically, I'm taking an integer pixel coordinate from 0 to 640,
converting it to a floating point number, dividing it by a floating
point 640, and multiplying that result by another small float, in this
case, 9.0e-6.  And the answer doesn't change with each increment of the
first number.  I've written a stand-alone test program doing just this
calculation and here are the
results:

Pixel_x = 100   Domain_x = 1.00000143051147460000
Pixel_x = 101   Domain_x = 1.00000143051147460000
Pixel_x = 102   Domain_x = 1.00000143051147460000
Pixel_x = 103   Domain_x = 1.00000143051147460000
Pixel_x = 104   Domain_x = 1.00000143051147460000
Pixel_x = 105   Domain_x = 1.00000143051147460000
Pixel_x = 106   Domain_x = 1.00000154972076420000
Pixel_x = 107   Domain_x = 1.00000154972076420000
Pixel_x = 108   Domain_x = 1.00000154972076420000
Pixel_x = 109   Domain_x = 1.00000154972076420000
Pixel_x = 110   Domain_x = 1.00000154972076420000
Pixel_x = 111   Domain_x = 1.00000154972076420000
Pixel_x = 112   Domain_x = 1.00000154972076420000
Pixel_x = 113   Domain_x = 1.00000154972076420000
Pixel_x = 114   Domain_x = 1.00000154972076420000
Pixel_x = 115   Domain_x = 1.00000166893005370000
Pixel_x = 116   Domain_x = 1.00000166893005370000
Pixel_x = 117   Domain_x = 1.00000166893005370000
Pixel_x = 118   Domain_x = 1.00000166893005370000
Pixel_x = 119   Domain_x = 1.00000166893005370000

If you're interested, I can post the code.  These results are far
grainier, numerically, than they hardware can provide.  Does anyone
understand what's going on?


Rich Wagner 


------------------------------------------------------------------------
------
ThinkGeek and WIRED's GeekDad team up for the Ultimate GeekDad Father's
Day Giveaway. ONE MASSIVE PRIZE to the lucky parental unit.  See the
prize list and enter to win: 
http://p.sf.net/sfu/thinkgeek-promo
_______________________________________________
MinGW-users mailing list
Min...@li...

This list observes the Etiquette found at
http://www.mingw.org/Mailing_Lists.
We ask that you be polite and do the same.  Disregard for the list
etiquette may cause your account to be moderated.

_______________________________________________
You may change your MinGW Account Options or unsubscribe at:
https://lists.sourceforge.net/lists/listinfo/mingw-users


SciSys UK Limited. Registered in England and Wales No. 4373530.
Registered Office: Methuen Park, Chippenham, Wiltshire SN14 0GB, UK.
 
* Before printing, please think about the environment.