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Date: Sun, 05 Mar 2006 14:58:33 +0100
From: Roberto Bagnara <bagnara AT cs DOT unipr DOT it>
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   "The Parma Polyhedra Library developers' list" <ppl-devel AT cs DOT unipr DOT it>
Subject: Re: Precision of doubles and stdio
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skaller wrote:
> On Sun, 2006-03-05 at 12:44 +0100, Roberto Bagnara wrote:
>> Tim Prince wrote:
>>  My 
>>> past reading of various relevant documents convinced me that digits 
>>> beyond the 17th in formatting of doubles are not required by any 
>>> standard to be consistent between implementations.  They have no useful 
>>> function, as 17 digits are sufficient to determine uniquely the 
>>> corresponding binary value in IEEE 754 format.
>> Thank you Tim.  We were unaware of this giant bug in the C standard.
>> All the best,
> 
> There is no bug in the C Standard. The C standard makes it 
> clear the accuracy of floating point operations is 
> implementation defined ,and the implementor may even say the 
> accuracy is undefined.

Which operations are you talking about?  I am not talking
about floating point operations.

> This is not a bug, it is the proper thing for a language
> standard. 

Call it the way you want: I call `buggy' a standard that
allows an invocation of

     printf("%.37g\n", d);

to silently ignore 20 or so significant digits (and apparently
for no good reason, by the way).  You can call it `bad design',
if you prefer.  Or `unfortunate legacy'.  You are of course
free to call it `good design' if you like it.
All the best,

     Roberto

-- 
Prof. Roberto Bagnara
Computer Science Group
Department of Mathematics, University of Parma, Italy
http://www.cs.unipr.it/~bagnara/
mailto:bagnara AT cs DOT unipr DOT it

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