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[Spock@SAMSON.CADR.DIALNET.SYMBOLICS.COM: Floating-point round-off problems.]
Date: Thu, 10 Aug 89 13:40:52 CDT
From: firstname.lastname@example.org (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?
+> (- 6 5.9) returns -> 0.099999905 instead of 0.1
+> --Mark Alexander
It looks like a problem common to more than Symbolics.
It's a general problem with conversion between decimal and binary
floating point. 1/10 can't be represented exactly in binary floating
point -- it's a repeating fraction (just like 1/3 is in decimal). The
closest approximation to 5.9 in binary floating point turns out to be
5.900000095367431640625 (see the documentation of the variable
*PRINT-EXACT-FLOAT-VALUE* to find out how to see the full value).
Austin Kyoto Common Lisp on a Sun 4-110 returns:
This is what Symbolics returns if you use double-floats. AKCL
presumably only has only one float format, double-float (understandable,
considering its C background, since pre-ANSI C favors use of doubles).