Re: calculating the power of a fixed point

From	glen herrmannsfeldt <gah@ugcs.caltech.edu>
Newsgroups	comp.programming, comp.lang.pl1
Subject	Re: calculating the power of a fixed point
Date	2011-10-01 10:56 +0000
Organization	Aioe.org NNTP Server
Message-ID	<j66rl4$4s1$1@speranza.aioe.org> (permalink)
References	(4 earlier) <slrn3vfsj7q32k.jl0.ike@sverige.freeshell.org> <lPednSOuSqKogODTnZ2dnUVZ_hednZ2d@earthlink.com> <j5kgme$rm8$1@dont-email.me> <j8-dnVxOZvxbe-DTnZ2dnUVZ_rqdnZ2d@earthlink.com> <c24870bd-913e-4dc0-a8b2-cd055f999f43@dm9g2000vbb.googlegroups.com>

Cross-posted to 2 groups.

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In comp.lang.pl1 Robin Vowels <robin.vowels@gmail.com> wrote:

(snip, someone, somewhere, wrote)
>> >>>>> What is the exact result of 1.6 * 1.6 (one decimal place) ?

>> >>>> The exact result is 2.56. Why would you drop one digit?

>> >>> Isn't that what fixed-point arithmetic is about? Having a fixed
>> >>> number of digits after the decimal (or binary) point?

One favorite PL/I example for many years is the expression 25+1/3.

The value, surprisingly enough, is 5.3333333333 (with the exact
number of 3's implementation dependent).  This is a strange
side-effect of the precision rules and the fact that constants
have the attributed in which they are written.

(For those worried about the answer, try 25+01/3 instead.)

Addition and subtraction give the number of digits after the
radix point as the maximum of the two operands, as one would
do in hand arithmetic.  Multiply gives the sum of the digits
after the radix point, again as for hand arithmetic.  (As with
integer multiply in other languages, digits may be lost in
overflow unless FIXEDOVERFLOW is enabled.)  

There is no easy answer for the scale factor in division, though.
Look it up in the manual, and you will find why the above expression
gives the value that it does.

-- glen

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calculating the power of a fixed point Keren Davidpur <kerenpur@gmail.com> - 2011-09-20 02:00 -0700
  Re: calculating the power of a fixed point "BartC" <bc@freeuk.com> - 2011-09-20 12:01 +0100
    Re: calculating the power of a fixed point "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> - 2011-09-20 14:36 +0200
      Re: calculating the power of a fixed point Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-09-21 03:52 +0100
        Re: calculating the power of a fixed point "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> - 2011-09-21 10:36 +0200
      Re: calculating the power of a fixed point Robin Vowels <robin.vowels@gmail.com> - 2011-09-23 04:03 -0700
        Re: calculating the power of a fixed point Ike Naar <ike@sverige.freeshell.org> - 2011-09-23 11:52 +0000
          Re: calculating the power of a fixed point Robin Vowels <robin.vowels@gmail.com> - 2011-09-23 08:59 -0700
            Re: calculating the power of a fixed point Ike Naar <ike@sverige.freeshell.org> - 2011-09-23 22:46 +0000
              Re: calculating the power of a fixed point Patricia Shanahan <pats@acm.org> - 2011-09-23 16:58 -0700
                Re: calculating the power of a fixed point "BartC" <bc@freeuk.com> - 2011-09-24 12:58 +0100
                Re: calculating the power of a fixed point Patricia Shanahan <pats@acm.org> - 2011-09-24 07:18 -0700
                Re: calculating the power of a fixed point Jongware <jongware@no-spam.plz> - 2011-09-26 11:03 +0200
                Re: calculating the power of a fixed point "BartC" <bc@freeuk.com> - 2011-09-26 10:40 +0100
                Re: calculating the power of a fixed point Robin Vowels <robin.vowels@gmail.com> - 2011-10-01 03:51 -0700
                Re: calculating the power of a fixed point Robin Vowels <robin.vowels@gmail.com> - 2011-10-01 03:44 -0700
                Re: calculating the power of a fixed point glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2011-10-01 10:56 +0000
                Re: calculating the power of a fixed point Robin Vowels <robin.vowels@gmail.com> - 2011-10-01 07:00 -0700
                Re: calculating the power of a fixed point glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2011-10-01 18:04 +0000
                Re: calculating the power of a fixed point Robin Vowels <robin.vowels@gmail.com> - 2011-10-01 16:27 -0700
                Re: calculating the power of a fixed point Robin Vowels <robin.vowels@gmail.com> - 2011-10-02 00:19 -0700
                Re: calculating the power of a fixed point Robin Vowels <robin.vowels@gmail.com> - 2011-10-02 00:24 -0700
  Re: calculating the power of a fixed point Dann Corbit <dcorbit@connx.com> - 2011-09-20 12:09 -0700
  Re: calculating the power of a fixed point Robin Vowels <robin.vowels@gmail.com> - 2011-09-22 03:21 -0700
  Re: calculating the power of a fixed point Robin Vowels <robin.vowels@gmail.com> - 2011-09-23 04:07 -0700
    Re: calculating the power of a fixed point Peter Flass <Peter_Flass@Yahoo.com> - 2011-09-23 08:17 -0400
      Re: calculating the power of a fixed point Robin Vowels <robin.vowels@gmail.com> - 2011-09-23 08:30 -0700
    Re: calculating the power of a fixed point glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2011-09-23 12:39 +0000
      Re: calculating the power of a fixed point Robin Vowels <robin.vowels@gmail.com> - 2011-09-23 08:44 -0700
        Re: calculating the power of a fixed point glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2011-09-23 17:07 +0000
          Re: calculating the power of a fixed point Robin Vowels <robin.vowels@gmail.com> - 2011-09-23 19:17 -0700

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