Path: utzoo!attcan!uunet!cs.utexas.edu!usc!snorkelwacker!apple!agate!usenet
From: deadman@garnet.berkeley.edu (Ben Haller)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Why can't the Mac add?
Message-ID: <1990Sep28.013600.598@agate.berkeley.edu>
Date: 28 Sep 90 01:36:00 GMT
Sender: usenet@agate.berkeley.edu (USENET Administrator)
Organization: Stick Software
Lines: 24

This topic is getting really old, but one more thing:  I've seen several
people so far say that the "correct" version of the loop
   for (x = 0.0 ; x != 10.0 ; x += 0.2)
is
   for (x = 0.0 ; x < 10.0 ; x += 0.2)
Now I don't claim to know much about floating point stuff, but it seems
to me that the internal representation of 0.2 could be either slightly
greater or slightly less than 0.2 (the reason why it isn't *exactly* 0.2
has been made quite clear, I think).  If the value used as 0.2 is slightly
less than 0.2, this loop will execute 51 times.  If the value used is
slightly greater, this loop will execute 50 times.  This is obviously
wrong.  What you need to do is either use an epsilon value, like:
  for (x = 0.0 ; abs(x - 10.0) > 0.1 ; x += 0.2)
or you need to use a constant that is not a multiple of 0.2 to test
against:
  for (x = 0.0 ; x < 9.9 ; x += 0.2)

(or, as has already been mentioned, there are various ways of doing the
equivalent loop with integers, although they are all slower than a pure
FP loop like the last one above)

Or am I wrong?

-Ben Haller (deadman@garnet.berkeley.edu)