Path: utzoo!attcan!uunet!tut.cis.ohio-state.edu!uc!cs.umn.edu!ux.acs!vw.acs.umn.edu!dhoyt
From: dhoyt@vw.acs.umn.edu
Newsgroups: comp.sys.mac.programmer
Subject: Re: Why can't the Mac add?
Message-ID: <2257@ux.acs.umn.edu>
Date: 24 Sep 90 18:52:04 GMT
Sender: news@ux.acs.umn.edu
Organization: University of Minnesota, Academic Computing Services
Lines: 16

>As you can see,  it ought to stop when x==10.0.  However, 
>it actually runs in an infinite loop.  This is because
>x never equals exactly 10.0.  The +0.2 always makes x
>equal to 9.999999 or 10.000001 or something.  Changing
>x from a float to a double makes x stay closer to the
>correct value, but it still isn't exactly correct, and
>as the loop runs longer, x gets off more and more.
>So my question is, why can't the Mac do simple math?

  0.2 is a continuing fraction in base 2.  Just like 1/3 can't be expressed
in base ten notation, 1/5, 1/10 and other numbers cann't be represented in base
2.  Some machines have some very special checks to handle 1/10th nicely, but
you should never count on it.  Real programmers use >=, even for integer loop
counters.  Paranoia, it's not just a state of mind, it's a job.

david paul hoyt | dhoyt@vx.acs.umn.edu | dhoyt@umnacvx.bitnet