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Mail Archives: djgpp/1999/04/13/18:36:51

From: no_mail_from AT use DOT net (Brian Chapman)
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Qustion on the casting of float's to int's...
Message-ID: <MPG.117d7c0c1fff4e9c989681@news.southwind.net>
References: <199904131434 DOT QAA03789 AT acp3bf DOT physik DOT rwth-aachen DOT de>
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Date: Tue, 13 Apr 1999 17:07:17 -0500
To: djgpp AT delorie DOT com
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Reply-To: djgpp AT delorie DOT com

Ahh! I see. I'll be sure to watch my step from now on.
Thank you very much! :-)


Previously, broeker AT physik DOT rwth-aachen DOT de says...
> You've just encountered a pitfall that surprised many beginners,
> before they learn how floating point numbers actually work, and what
> their limitations are. The basic problem is that '0.2' is not exactly
> representable by *any* floating point value, in a PC.  Instead, you'll
> get something like 0.1999999.... or 0.20000001...  In the case at
> hand, it's the first kind.
> 
> Adding up several of these values, you'll see that you won't ever
> reach 1.0 exactly. Instead, you get 0.999999999. Because the FPU has
> some hidden extra precision, the problem actually does not happen
> around a sum of 1.0, but only when it's reached 5.0 or so.
> 
> This whole issue is summed up in a nice quote:
> 
> 	In computers, 10 times 0.1 is hardly ever 1.0
> 
> > i'm still new to C, but what is the compiler (gcc 2.8.1) doing???
> > why does (int)2.0=2 yet (int)8.0=7??? I have always assumed in the
> > back of my mind that typecasting floats to ints just rounded down to
> > the nearest whole number (ie: truncated the fraction).
> 
> Typecasting does that. But *printing* doesn't truncate, it really
> rounds. The real floating point value is 7.9999999.... which will be
> printed as '8.0', but the truncated value is 7.
> 
> 
> --
> Hans-Bernhard Broeker (broeker AT physik DOT rwth-aachen DOT de)
> Even if all the snow were burnt, ashes would remain.
> 

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