From: Tim Hollebeek <tim@hollebeek.com>
To: Paul Koning <pkoning@equallogic.com>
Cc: dewar@gnat.com, geoffk@geoffk.org, lars@nocrew.org, gcc@gcc.gnu.org
Subject: Re: Comparing doubles
Date: Sun, 07 Jul 2002 18:06:00 -0000 [thread overview]
Message-ID: <20020707161828.A13448@hollebeek.com> (raw)
In-Reply-To: <15656.15769.42000.732149@gargle.gargle.HOWL>
On Sun, Jul 07, 2002 at 09:09:45AM -0400, Paul Koning wrote:
>
> As for compare, I don't claim to know the intricacies of IEEE NaN and
> all that, but surely on older float formats at least, compare is
> indeed internally just a subtract and test for zero?
The original suggestion, which seems to be missed by many, was
subtract _doubles_ and compare to _float_ zero. Assuming that wasn't
an mistake, it is far, far different from standard equality (due to
the loss of precision from truncation).
Truncation before compare allows {a==b, b==c, c!=a}, and all other
sorts of unreasonable things. Regardless of whether the standard
strictly requires it or not, it is reasonable to expect equality to be
an equivalence operation.
-Tim
next prev parent reply other threads:[~2002-07-07 20:47 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2002-07-06 10:58 Robert Dewar
2002-07-07 9:09 ` Paul Koning
2002-07-07 18:06 ` Tim Hollebeek [this message]
2002-07-08 2:19 ` Lars Brinkhoff
-- strict thread matches above, loose matches on Subject: below --
2002-07-07 21:37 Robert Dewar
2002-07-07 22:27 ` Tim Hollebeek
2002-07-07 9:20 Robert Dewar
2002-07-07 9:17 Robert Dewar
2002-07-08 3:10 ` Kai Henningsen
2002-07-06 10:28 Robert Dewar
2002-07-01 11:16 Lars Brinkhoff
2002-07-01 11:55 ` Geoff Keating
2002-07-01 23:24 ` Lars Brinkhoff
2002-07-02 7:48 ` Lars Brinkhoff
2002-07-01 12:13 ` Tim Hollebeek
2002-07-01 12:18 ` Alan Lehotsky
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