[Spock@SAMSON.CADR.DIALNET.SYMBOLICS.COM: Floating-point round-off problems.]

["Mr. Spock" <Spock%samson.cadr.dialnet.symbolics.com@RELAY.CS.NET>]

    Date: Thu, 10 Aug 89 17:16 CDT
    From: dmitchell@backus.trc.amoco.com (Donald H. Mitchell)

	Date: Thu, 10 Aug 89 13:40:52 CDT
	From: mac%cadillac.cad.mcc.com@mcc.com (Mac Michaels)

	+>  Date: Thu, 10 Aug 89 08:00 PDT
	+>  From: Spock@SAMSON.CADR.DIALNET.SYMBOLICS.COM (Mr. Spock)
	+>  
	+>  Does anyone have a hack to correct for the losing floating-point math on
	+>  these Symbolics machines?
	+>  
	+>  Consider:
	+>  
	+>      (- 6 5.9) returns -> 0.099999905 instead of 0.1
	+>  
	+>  Thanks,
	+>  --Mark Alexander

	It looks like a problem common to more than Symbolics.  

	Austin Kyoto Common Lisp on a Sun 4-110 returns:

	0.09999999999999964

	Extension Language Kit (a superset of Scheme) on a Sun 3 returns:

	0.0999999999999996


	USPS: Mac Michaels,   3500 W.Balcones Center Dr.,   Austin,TX 78759
	ARPA: mac@mcc.com
	UUCP: {uunet,harvard,gatech,pyramid}!cs.utexas.edu!milano!cadillac!mac 

    Oh my GOD!  Look what happens with Sun's C:

Gag, barf!

    Float:   0.099999904632568; 
    double:  0.09999999999999964472863211995; 
    immediate:  0.09999999999999964472863211995

    It must be a conspiracy! Who's stealing our floating points!!??

Yes it must.  The russians?

    Here's the source, see if you can figure it out!
    #include <stdio.h>
    main()

    { float a,b;
      double c,d;

      a = 6.0;
      c = 6.0;
      d = 5.9;
      b = 5.9;
      a = a - b;
      c = c - d;
 
      printf ( "\n\nFloat: %19.14g; double: %32.29g; immediate: %32.29g\n\n",
    a ,  c, 
		    (6.0 - 5.9));
      }

Thank-you one and all for your gentle rebufs concerning my naive
question.  I'll be chuckling over Mitchell's response all day.
--Mark Alexander