Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!cs.utexas.edu!usc!apple!das
From: das@Apple.COM (David Shayer)
Newsgroups: comp.sys.mac.programmer
Subject: Why can't the Mac add?
Message-ID: <45060@apple.Apple.COM>
Date: 23 Sep 90 00:02:26 GMT
Organization: Apple Computer Inc., Cupertino, CA
Lines: 29

I was running this simple program.

main ()
{
	float	x; 
	
	for (x=0.0;x!=10.0;x+=0.2)
		printf ("x=%f \n",x);
}

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?
What's going on here?  Is SANE insane?

I have a Mac IIci, so I have a 68882 FPU.  This program
fails in both Think and MPW C.  I didn't compile with any
special FPU options on.  A friend ran it on a
PC clone, and it failed in the same way there.

Please enlighten me.

David