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Re: [Help-gsl] Double vs Long Double

From: Martin Jansche
Subject: Re: [Help-gsl] Double vs Long Double
Date: Sat, 29 Sep 2007 13:03:13 -0400

Most parts of the public API of GSL use double precision, so you're
limited to 53 significant binary digits.  That means 53*log(10)/log(2)
~= 15.95 decimal digits.  If you want to interpret results in decimal,
you should print them out with no more than 15 (or more properly the
value of DBL_DIG) significant decimal digits, e.g. using the format
string "%.15g".  The only reason to use more decimal digits is if you
want safe round-trip conversion from binary to decimal and back.  If
you want to make sure you got exactly the same double value, print it
out in hex ("%a"), or binary for debugging.  Long story short, you're
getting exactly the same result as in the documentation.

-- mj

On 9/28/07, address@hidden <address@hidden> wrote:
> I just got the  GSL book today and immediately
> tried the sample program on page 9.
> I did not get the exact same answer shown in the book:
>       JO(5) = -1.775967713143382920e-01
> I got J)(5) = -1.775967713143382642e-01
>                                 ^^^
> I'm running 64-bit AMD OpenBSD 4.1 on AM2 Athlon 64.
> gcc (GCC) 3.3.5 (propolice)
> Copyright (C) 2003 Free Software Foundation, Inc.
> This is free software; see the source for copying conditions.  There is NO
> FP computation is done using MMX registers except for long double which
> is done using the 8087 registers. I am now working on a version of GSL
> compiled using long double to force use of the 80-bit FP data format
> and see if I get the result listed in the book.
> Comments?
> Thanks,
> Dave Feustel
> --
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