Re: complex number in struct form in C

From	glen herrmannsfeldt <gah@ugcs.caltech.edu>
Newsgroups	comp.lang.c
Subject	Re: complex number in struct form in C
Date	2014-12-28 07:17 +0000
Organization	Aioe.org NNTP Server
Message-ID	<m7oar1$no0$1@speranza.aioe.org> (permalink)
References	(4 earlier) <m76uab$bi7$1@dont-email.me> <cfod3vFtubaU1@mid.individual.net> <f03e9ap9a4qac9er1qb2hi4dtb5nq00978@4ax.com> <62472149-c09e-42d6-9789-3d62b9b3cdf0@googlegroups.com> <549DC451.1070801@verizon.net>

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James Kuyper <jameskuyper@verizon.net> wrote:
> On 12/26/2014 02:34 PM, asetofsymbols@gmail.com wrote:
>> Why use float, double, long double? For me it is better fixed 
>> point float... I not understand how people use float not fixed 
>> point could be sure all ok in financials calculus 

> I work primarily in scientific data processing. For most intermediate
> calculations, we use double, but when writing to a file, we'll often use
> float, if it has sufficient precision, which is often the case. The
> images we're working on have as resolution of 250 meters, a float has
> sufficient precision to store the latitude, longitude, solar zenith and
> azimuth angles, and unit vectors pointing at the moon and the sun.

For a large variety of scientific data processing, double precision
should be used for intermediate calculation when single precision
results are needed.  

It is very easy for precision to be lost when subtracting two values
of similar magnitude in an intermediate value. For example, matrix
multiplication involves a sum over products of matrix elements for 
each term in the product matrix. It isn't hard to find cases where
most of the bits are lost in that calculation.

One of the worst problems, though one that often enough comes up
in high school math class, is matrix inversion.  Only for very small
matrices is double precision likely to generate even single precision
results.

> However, a large fraction of the data produced by the programs I'm
> responsible for is stored as scaled integers, usually 16 bits. The value
> represented is scale_factor*scaled_integer+offset, where scale factor
> and offset are chosen at run time to spread out the valid range of
> represented values as evenly as possible from the range scaled integer
> values from 0 to 65535 (or, for image data, 0 to 32767 - values over
> 32768 are reserved for special uses). Conceptually, this isn't horribly
> different from fixed point floats, which are conceptually equivalent to
> choosing an offset of 0 and a scale factor which is a power of 10 (or
> 2). However, the scale and offset approach allows more precise matching
> of the range of the data to the range of representable integers.

Much of numerical analysis is knowing where algorithm can go wrong,
and adjusting as appropriate.  For example, LU decomposition can
be used in many cases that would be more obvious done with matrix
inversion.

-- glen

Thread

complex number in struct form in C Dexterr <dejangru@gmail.com> - 2014-12-21 07:20 -0800
  Re: complex number in struct form in C James Kuyper <jameskuyper@verizon.net> - 2014-12-21 11:30 -0500
    Re: complex number in struct form in C Dexterr <dejangru@gmail.com> - 2014-12-21 08:35 -0800
      Re: complex number in struct form in C "Osmium" <r124c4u102@comcast.net> - 2014-12-21 10:54 -0600
        Re: complex number in struct form in C James Kuyper <jameskuyper@verizon.net> - 2014-12-21 12:00 -0500
          Re: complex number in struct form in C "Osmium" <r124c4u102@comcast.net> - 2014-12-21 11:08 -0600
            Re: complex number in struct form in C Barry Schwarz <schwarzb@dqel.com> - 2014-12-21 10:12 -0800
              Re: complex number in struct form in C "Osmium" <r124c4u102@comcast.net> - 2014-12-21 13:22 -0600
                Re: complex number in struct form in C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2014-12-21 22:21 +0000
              Re: complex number in struct form in C asetofsymbols@gmail.com - 2014-12-26 11:34 -0800
                Re: complex number in struct form in C "Osmium" <r124c4u102@comcast.net> - 2014-12-26 14:10 -0600
                Re: complex number in struct form in C glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-12-28 07:04 +0000
                Re: complex number in struct form in C BartC <bc@freeuk.com> - 2014-12-28 13:14 +0000
                Re: complex number in struct form in C Robert Wessel <robertwessel2@yahoo.com> - 2014-12-28 10:45 -0600
                Re: complex number in struct form in C Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-12-28 08:56 -0800
                Re: complex number in struct form in C glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-12-29 10:10 +0000
                Re: complex number in struct form in C Kaz Kylheku <kaz@kylheku.com> - 2014-12-29 19:40 +0000
                Re: complex number in struct form in C glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-12-29 20:54 +0000
                Re: complex number in struct form in C BartC <bc@freeuk.com> - 2014-12-29 21:55 +0000
                Re: complex number in struct form in C glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-12-30 03:07 +0000
                Re: complex number in struct form in C BartC <bc@freeuk.com> - 2014-12-30 10:51 +0000
                Re: complex number in struct form in C Rosario193 <Rosario@invalid.invalid> - 2014-12-30 12:15 +0100
                Re: complex number in struct form in C Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-12-30 03:31 -0800
                Re: complex number in struct form in C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2014-12-30 12:17 +0000
                Re: complex number in struct form in C BartC <bc@freeuk.com> - 2014-12-30 13:18 +0000
                Re: complex number in struct form in C Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-12-30 05:28 -0800
                Re: complex number in struct form in C BartC <bc@freeuk.com> - 2014-12-30 13:53 +0000
                Re: complex number in struct form in C "Osmium" <r124c4u102@comcast.net> - 2014-12-30 08:53 -0600
                Re: complex number in struct form in C "Chris M. Thomasson" <no@spam.invalid> - 2014-12-30 20:05 -0800
                Re: complex number in struct form in C Jeff Higgins <jeff@invalid.invalid> - 2014-12-30 17:40 -0500
                Re: complex number in struct form in C Tim Rentsch <txr@alumni.caltech.edu> - 2015-03-24 08:58 -0700
                Re: complex number in struct form in C Les Cargill <lcargill99@comcast.com> - 2015-03-24 19:41 -0500
                Re: complex number in struct form in C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2014-12-30 14:59 +0000
                Re: complex number in struct form in C David Thompson <dave.thompson2@verizon.net> - 2015-01-12 15:33 -0500
                Re: complex number in struct form in C glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-01-12 22:51 +0000
                Re: complex number in struct form in C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-01-12 23:34 +0000
                Re: complex number in struct form in C James Kuyper <jameskuyper@verizon.net> - 2014-12-26 15:25 -0500
                Re: complex number in struct form in C glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-12-28 07:17 +0000
                Re: complex number in struct form in C BartC <bc@freeuk.com> - 2014-12-26 20:49 +0000
                Re: complex number in struct form in C asetofsymbols@gmail.com - 2014-12-27 00:22 -0800
                Re: complex number in struct form in C "Osmium" <r124c4u102@comcast.net> - 2014-12-27 08:25 -0600
                Re: complex number in struct form in C Geoff <geoff@invalid.invalid> - 2014-12-27 10:20 -0800
                Re: complex number in struct form in C "Osmium" <r124c4u102@comcast.net> - 2014-12-27 13:21 -0600
                Re: complex number in struct form in C Geoff <geoff@invalid.invalid> - 2014-12-27 13:11 -0800
                Re: complex number in struct form in C "Osmium" <r124c4u102@comcast.net> - 2014-12-27 15:36 -0600
                Re: complex number in struct form in C Geoff <geoff@invalid.invalid> - 2014-12-27 14:54 -0800
                Re: complex number in struct form in C Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-12-27 15:45 -0800
                Re: complex number in struct form in C Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-12-27 13:40 -0800
                Re: complex number in struct form in C James Kuyper <jameskuyper@verizon.net> - 2014-12-28 06:50 -0500
                Re: complex number in struct form in C Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-12-28 04:25 -0800
                Re: complex number in struct form in C James Kuyper <jameskuyper@verizon.net> - 2014-12-28 06:44 -0500
                Re: complex number in struct form in C Richard Damon <Richard@Damon-Family.org> - 2014-12-28 08:33 -0500
                Re: complex number in struct form in C James Kuyper <jameskuyper@verizon.net> - 2014-12-27 11:46 -0500
                Re: complex number in struct form in C glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-12-28 07:28 +0000
                Re: complex number in struct form in C Kaz Kylheku <kaz@kylheku.com> - 2014-12-27 17:23 +0000
            Re: complex number in struct form in C James Kuyper <jameskuyper@verizon.net> - 2014-12-21 14:37 -0500
          Re: complex number in struct form in C Keith Thompson <kst-u@mib.org> - 2014-12-21 12:37 -0800
          Re: complex number in struct form in C glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-12-22 00:54 +0000
      Re: complex number in struct form in C James Kuyper <jameskuyper@verizon.net> - 2014-12-21 11:54 -0500
      Re: complex number in struct form in C Vincenzo Mercuri <nomail@yahoo.it> - 2014-12-21 18:07 +0100
        Re: complex number in struct form in C Dexterr <dejangru@gmail.com> - 2014-12-21 10:37 -0800
          Re: complex number in struct form in C "Chris M. Thomasson" <no@spam.invalid> - 2014-12-21 10:55 -0800
          Re: complex number in struct form in C raltbos@xs4all.nl (Richard Bos) - 2014-12-22 12:15 +0000
            Re: complex number in struct form in C Dexterr <dejangru@gmail.com> - 2014-12-22 05:51 -0800
              Re: complex number in struct form in C BartC <bc@freeuk.com> - 2014-12-22 14:01 +0000
                Re: complex number in struct form in C Keith Thompson <kst-u@mib.org> - 2014-12-22 08:02 -0800
            Re: complex number in struct form in C Keith Thompson <kst-u@mib.org> - 2014-12-22 08:03 -0800
    Re: complex number in struct form in C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2014-12-21 22:27 +0000
  Re: complex number in struct form in C "Chris M. Thomasson" <no@spam.invalid> - 2014-12-21 10:51 -0800
    Re: complex number in struct form in C Dexterr <dejangru@gmail.com> - 2014-12-21 11:14 -0800
  Re: complex number in struct form in C Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-12-21 17:17 -0800

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