Re: storing and retrieving a double may show a difference?

From	Ben Bacarisse <ben.usenet@bsb.me.uk>
Newsgroups	comp.lang.c
Subject	Re: storing and retrieving a double may show a difference?
Date	2018-12-23 12:38 +0000
Organization	A noiseless patient Spider
Message-ID	<87va3kifk0.fsf@bsb.me.uk> (permalink)
References	(1 earlier) <0711f24a-9572-4e17-b51b-fe454cd26b23@googlegroups.com> <EADTD.126137$e%3.117666@fx42.iad> <7c943b6c-b1ff-48f0-93f8-c6615f4e600a@googlegroups.com> <9u8u1etbu8vkvenav2lr3fjji3cks4e9aq@4ax.com> <e69a2fe3-2707-4959-b5b0-596dc79c0da4@googlegroups.com>

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chanrasjid@gmail.com writes:

> On Sunday, December 23, 2018 at 2:02:26 PM UTC+8, Barry Schwarz wrote:
>> On Sat, 22 Dec 2018 20:10:35 -0800 (PST), chanrasjid@gmail.com wrote:
>> 
>> >On Sunday, December 23, 2018 at 11:49:00 AM UTC+8, Richard Damon wrote:
>> >> On 12/22/18 10:30 PM, chanrasjid@gmail.com wrote:
>> >> > Just a simple C question now.
>> >> > 
>> >> > In C, is it a proper programming thing to do a comparison of
>> >> > values for x, a double as: If (x == 0.0) or if (x == 1.0).
>> >> > 
>> >> > Ben just mentioned ==, !=, etc are "just" comparisons, so it
>> >> > seems OK to compare floating values.

Provided you know what you are doing.  The point being made in the post
to which I replied was that you often don't want exact comparison
because floating point calculations usually accrue errors.

<snip>
> I think yes. But how do you "instinctively" know 1.23 cannot be stored
> as exact in binary?

Binary floating point can store integers with a fractional part that is
made up of a sum of halves, quarters, eighths and so on.  So sums of .5,
.25, .125 and so on, but not 1.23.

-- 
Ben.

Thread

storing and retrieving a double may show a difference? chanrasjid@gmail.com - 2018-12-20 23:25 -0800
  Re: storing and retrieving a double may show a difference? fir <profesor.fir@gmail.com> - 2018-12-21 00:56 -0800
    Re: storing and retrieving a double may show a difference? chanrasjid@gmail.com - 2018-12-21 07:02 -0800
      Re: storing and retrieving a double may show a difference? Bart <bc@freeuk.com> - 2018-12-21 15:33 +0000
        Re: storing and retrieving a double may show a difference? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2018-12-21 11:16 -0500
          Re: storing and retrieving a double may show a difference? chanrasjid@gmail.com - 2018-12-21 09:05 -0800
            Re: storing and retrieving a double may show a difference? fir <profesor.fir@gmail.com> - 2018-12-21 09:43 -0800
            Re: storing and retrieving a double may show a difference? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2018-12-21 12:57 -0500
            Re: storing and retrieving a double may show a difference? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-12-21 20:06 +0000
      Re: storing and retrieving a double may show a difference? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-12-21 17:10 +0000
        Re: storing and retrieving a double may show a difference? fir <profesor.fir@gmail.com> - 2018-12-21 09:17 -0800
          Re: storing and retrieving a double may show a difference? fir <profesor.fir@gmail.com> - 2018-12-21 09:20 -0800
          Re: storing and retrieving a double may show a difference? fir <profesor.fir@gmail.com> - 2018-12-21 09:24 -0800
        Re: storing and retrieving a double may show a difference? chanrasjid@gmail.com - 2018-12-21 21:46 -0800
        Re: storing and retrieving a double may show a difference? Manfred <noname@add.invalid> - 2018-12-22 21:41 +0100
          Re: storing and retrieving a double may show a difference? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-12-22 21:08 +0000
            Re: storing and retrieving a double may show a difference? Manfred <noname@add.invalid> - 2018-12-23 20:34 +0100
              Re: storing and retrieving a double may show a difference? Tim Rentsch <txr@alumni.caltech.edu> - 2018-12-28 09:14 -0800
          Re: storing and retrieving a double may show a difference? chanrasjid@gmail.com - 2018-12-22 19:16 -0800
      Re: storing and retrieving a double may show a difference? fir <profesor.fir@gmail.com> - 2018-12-21 09:12 -0800
      Re: storing and retrieving a double may show a difference? James Kuyper <jameskuyper@alumni.caltech.edu> - 2018-12-21 09:23 -0800
        Re: storing and retrieving a double may show a difference? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-12-21 20:03 +0000
          Re: storing and retrieving a double may show a difference? James Kuyper <jameskuyper@alumni.caltech.edu> - 2018-12-22 00:10 -0500
            Re: storing and retrieving a double may show a difference? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-12-22 11:06 +0000
  Re: storing and retrieving a double may show a difference? mark.bluemel@gmail.com - 2018-12-21 01:00 -0800
    What type of problem is it? (Was: storing and retrieving a double may show a difference?) gazelle@shell.xmission.com (Kenny McCormack) - 2018-12-21 09:10 +0000
      Re: What type of problem is it? (Was: storing and retrieving a double may show a difference?) mark.bluemel@gmail.com - 2018-12-21 01:33 -0800
  Re: storing and retrieving a double may show a difference? Bart <bc@freeuk.com> - 2018-12-21 10:16 +0000
  Re: storing and retrieving a double may show a difference? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-12-21 11:25 +0000
  Re: storing and retrieving a double may show a difference? James Kuyper <jameskuyper@alumni.caltech.edu> - 2018-12-21 08:06 -0500
    Re: storing and retrieving a double may show a difference? chanrasjid@gmail.com - 2018-12-21 07:13 -0800
      Re: storing and retrieving a double may show a difference? James Kuyper <jameskuyper@alumni.caltech.edu> - 2018-12-21 09:10 -0800
    Re: storing and retrieving a double may show a difference? Tim Rentsch <txr@alumni.caltech.edu> - 2018-12-28 08:16 -0800
  Re: storing and retrieving a double may show a difference? chanrasjid@gmail.com - 2018-12-21 08:09 -0800
  Re: storing and retrieving a double may show a difference? "Chris M. Thomasson" <invalid_chris_thomasson@invalid.invalid> - 2018-12-22 17:23 -0800
    Re: storing and retrieving a double may show a difference? "Chris M. Thomasson" <invalid_chris_thomasson@invalid.invalid> - 2018-12-22 17:58 -0800
  Re: storing and retrieving a double may show a difference? chanrasjid@gmail.com - 2018-12-22 19:30 -0800
    Re: storing and retrieving a double may show a difference? Richard Damon <Richard@Damon-Family.org> - 2018-12-22 22:48 -0500
      Re: storing and retrieving a double may show a difference? chanrasjid@gmail.com - 2018-12-22 20:10 -0800
        Re: storing and retrieving a double may show a difference? "Chris M. Thomasson" <invalid_chris_thomasson@invalid.invalid> - 2018-12-22 20:41 -0800
        Re: storing and retrieving a double may show a difference? Barry Schwarz <schwarzb@dqel.com> - 2018-12-22 22:02 -0800
          Re: storing and retrieving a double may show a difference? chanrasjid@gmail.com - 2018-12-23 02:43 -0800
            Re: storing and retrieving a double may show a difference? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-12-23 12:38 +0000
              Re: storing and retrieving a double may show a difference? chanrasjid@gmail.com - 2018-12-23 05:12 -0800
                Re: storing and retrieving a double may show a difference? fir <profesor.fir@gmail.com> - 2018-12-23 05:32 -0800
                Re: storing and retrieving a double may show a difference? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-12-23 14:55 +0000
            Re: storing and retrieving a double may show a difference? Richard Damon <Richard@Damon-Family.org> - 2018-12-23 07:43 -0500
    Re: storing and retrieving a double may show a difference? Manfred <noname@add.invalid> - 2018-12-23 20:42 +0100

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