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Re: [Fwd: Re: ruleutils with pretty-print option]

From: Manfred Koizar <mkoi-pg(at)aon(dot)at>
To: Andreas Pflug <pgadmin(at)pse-consulting(dot)de>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-patches(at)postgresql(dot)org
Subject: Re: [Fwd: Re: ruleutils with pretty-print option]
Date: 2003-07-31 16:01:21
Message-ID: 5vdiivkpp59flilu0hvlc9nec31fe07ipr@4ax.com (view raw or flat)
Thread:
Lists: pgsql-patches
On Thu, 31 Jul 2003 16:30:17 +0200, Andreas Pflug
<pgadmin(at)pse-consulting(dot)de> wrote:
>Well, to me it's not well-known that floating-point addition is not 
>associative

This is a case of theory vs. practice mismatch:  In theory addition is
associative, in practice there is only limited storage available for a
floating-point number.  Let's do an example with 3 significant decimal
digits:

    a = 1000        internal representation: 1.00e3
    b =    1        internal representation: 1.00e0
    a + b = 1.001e3

which cannot be represented in our system, so it is rounded to 1.00e3
and we get

    a + b = 1000
    a + b + b + b + b + b + b + b + b + b + b = 1000

when evaluated left to right, but

    a + (b + b + b + b + b + b + b + b + b + b) = 
    1.00e3 + 1.00e1 = 1.01e3 = 1001

Servus
 Manfred

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Subject: Re: [Fwd: Re: ruleutils with pretty-print option]
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