Re: double value 10.20 -10.00 = ?? HELP!

[Brad Wilson - BTWeb Enterprises]

This appears to be a case of a phenomenon called "Subtractive
Cancellation" - 2nd year computer science mathematics stuff.  When a
floating point number is subtracted from another on a binary machine, the
error is many times greater than when performing any of the other three
basic arithmetic operations on two floating point numbers.  Having said
that, this case is particularly severe, as subtractive cancellation is
usually only this noticable when two very large floating point numbers are
subtracted from eachother to give a very small result.  Nevertheless, it
appears to explain what is happening here.

I do remember being taught ways in which you can get around it and perform
very accurate subtractions using floating point numbers (involving logs and
exp and maybe even some trigonometric functions), but I'd have to delve into
the old notes to find out, which I'm not likely to do  ;-)

Hopefully I've helped shed some light on *why* this is happening, but it
doesnt help in solving it I'm afraid.

Brad Wilson
---------------------------------------------
BT Web Enterprises
5/44 Park Street North
Woodville Park SA 5011
tel: (08) 8244 7760
fax: (08) 8244 7761
mob: 0419 848 707
email: [EMAIL PROTECTED]
---------------------------------------------

----- Original Message -----
From: "paul" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Friday, December 22, 2000 11:09 PM
Subject: Re: double value 10.20 -10.00 = ?? HELP!


> ummmm.. Finite precision variables????
>
> > I have the same problem with C++ program!
> > I compiled with Borland C++ 5.5 compiler.
> >
> >         float a = 1000000.20;
> >         float b = 1000000.00;
> >         float c = a - b;
> >
> > Result of c is 0.1875!!!!!!!!
> > ????
> >
> > We tried this on the same machine but on Linux operating system. Same
> > problem with Java and C++ code!
> >
> > Nobody understand why is this hapening!
> >
> > Reinkarnation of old Pentium bug?
> >
> > Regards,
> >
>
>
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