Re: printf and long double

[Chrysostomos Nanakos <nanakos@wired-net.gr>]

  On 4/10/2010 3:03 μμ, Bogdan Cristea wrote:
> On Monday 04 October 2010 14:54:46 you wrote:
>> I wish I could get 24 decimal digits precision with the current long
>> double (80-bit). I can't get more than 15 digits even in a simple division.
>> I think that the extended-precision can't give more than 19 decimal
>> digits of precision (log(10,2^64)=19.2). The quadruple precision can
>> give at most 34 decimal digits (log(10,2^113)=34).
>>
>> Please consider the example below:
>> --------------------------------------------------
>> long double a = 1.0L/7.0L;
>> printf("%Lf\n",a);
>> --------------------------------------------------
>>
>> Can't get more than 15 digits precision....-:(
>> Is this a compiler problem or libc's problem? In x86_64 machines the
>> long double values supposed to give at least 106-bit precision even when
>> implemented in software. Is gcc compliant with the quadruple notation?
> Have you tried to specify manually the number of digits after the decimal
> point ?
>
> 	long double a = 1.0L/7.0L;
> 	printf("%'.100Lf\n",a);
>
> On a 64 bits machine the output is:
>
> 0.1428571428571428571409210675491330277964152628555893898010253906250000000000000000000000000000000000
>
> regards
> Bogdan
Of course I did. But the result you get is the wrong one after the first 
20 decimal point digits. The right result is given below with a high 
precision arithmetic library or with Mathematica or Matlab. And the 
three results are the same, but not the given one above which is the 
same for my machine.

0.14285714285714285714285714285714285714285714285714285714285714285714\
28571428571428571428571428571429
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