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mismatch between Perl 5.6 and Perl 5.8 in printing high precision values.
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mismatch between Perl 5.6 and Perl 5.8 in printing high precision values. vivekanand.naik 04-02-2008
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Posted by vivekanand.naik on April 2, 2008, 6:21 am
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 printf("%.32g\n",0.99999999976716936);

Perl 5.6.1 output:
0.99999999976716936 --> GOOD

Perl 5.8.6 output:
0.99999999976716925 --> ERROR


Any reason for such mismatch ?
Please let me know how to avoid that or any alternative approach if
any.

Thanks
Vivek

Posted by RedGrittyBrick on April 2, 2008, 7:02 am
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 vivekanand.naik@gmail.com wrote:
> printf("%.32g\n",0.99999999976716936);
>
> Perl 5.6.1 output:
> 0.99999999976716936 --> GOOD
>
> Perl 5.8.6 output:
> 0.99999999976716925 --> ERROR
>
>
> Any reason for such mismatch ?
> Please let me know how to avoid that or any alternative approach if
> any.
>

print Math::BigFloat->new("0.99999999976716936")->bstr();


--
RGB

Posted by szr on April 2, 2008, 11:53 am
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RedGrittyBrick wrote:
> vivekanand.naik@gmail.com wrote:
>> printf("%.32g\n",0.99999999976716936);
>>
>> Perl 5.6.1 output:
>> 0.99999999976716936 --> GOOD
>>
>> Perl 5.8.6 output:
>> 0.99999999976716925 --> ERROR
>>
>>
>> Any reason for such mismatch ?
>> Please let me know how to avoid that or any alternative approach if
>> any.
>>
>
> print Math::BigFloat->new("0.99999999976716936")->bstr();


The version of Math::BigFloat that shipped with 5.6.1, which the op
seems to need, doesn't seem to support ->bstr()

$ perlall -MMath::BigFloat -e 'print
Math::BigFloat->new("0.99999999976716936")->bstr(), "\n";'
<perl5.10.0> 0.99999999976716936
<perl5.8.8 > 0.99999999976716936
<perl5.8.2 > 0.99999999976716936
<perl5.8.0 > 0.99999999976716936
Can't locate object method "bstr" via package "Math::BigFloat" (perhaps
you forgot to load "Math::BigFloat"?) at -e line 1.
<perl5.6.1 >


Perhaps ->ffround(17) could be used instead?

$ perl5.6.1 -MMath::BigFloat -e 'print
Math::BigFloat->new("0.99999999976716936")->ffround(-17), "\n";'
+99999999976716936E-17

You could always update your Math::* modules if you must use 5.6.1,
else, it would better to just install a newer Perl :-)

--
szr



Posted by Mirco Wahab on April 2, 2008, 7:18 am
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vivekanand.naik@gmail.com wrote:
> printf("%.32g\n",0.99999999976716936);
>
> Perl 5.6.1 output:
> 0.99999999976716936 --> GOOD
>
> Perl 5.8.6 output:
> 0.99999999976716925 --> ERROR
> Any reason for such mismatch ?
> Please let me know how to avoid that or any alternative approach if
> any.

This is in the interval from 1e-16 to 1e-15, which
is the accuracy of a IEEE 8 byte double in this
number range (52 bits for fraction). Seems to
be ok ==> 1 / (2^52).
The correct result (above) needs (imho) longer
than 8 byte doubles, like

perl -MMath::BigFloat -e'print
Math::BigFloat->new("0.99999999976716936")->bstr()'


Regards

M.


Posted by sisyphus on April 2, 2008, 9:46 am
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On Apr 2, 9:21=A0pm, vivekanand.n...@gmail.com wrote:
> printf("%.32g\n",0.99999999976716936);
>
> Perl 5.6.1 output:
> 0.99999999976716936 =A0--> GOOD
>
> Perl 5.8.6 output:
> 0.99999999976716925 =A0--> ERROR
>
> Any reason for such mismatch ?

I believe it's a bug in perl 5.8 (which has been carried through to
perl 5.10):

C:\_32\pscrpt>perl -e "printf(\"%.32g\n\",0.99999999976716936);"
0.99999999976716925

C:\_32\pscrpt>perl -e "printf(\"%.32g\n\",9.9999999976716936e-1);"
0.99999999976716936

Since 0.99999999976716936 =3D=3D 9.9999999976716936e-1, I can think of no
good reason that that those 2 one-liners should produce different
output - and I believe the first one liner produces an incorrect
result.

I'll submit a bug report about this - unless someone can convince me
that it's *not* a bug (or unless someone else wants to submit the
report).

Cheers,
Rob

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