Hi,
On Tue, Oct 25, 2011 at 11:14 AM, Pauli Virtanen <p...@iki.fi> wrote:
> 25.10.2011 19:45, Matthew Brett kirjoitti:
> [clip]
>>> or, in case the platform doesn't have powl:
>>>
>>> long double x;
>>> x = pow(2, 64);
>>> x -= 1;
>>> printf("%g %Lg\n", (double)x, x);
>>
>> Both the same as numpy:
>>
>> [mb312@jerry ~]$ gcc test.c
>> test.c: In function 'main':
>> test.c:5: warning: incompatible implicit declaration of built-in function
>> 'powl'
>> [mb312@jerry ~]$ ./a.out
>> 1.84467e+19 3.68935e+19
>
> This result may indicate that it's the *printing* of long doubles that's
> broken. Note how the value cast as double prints the correct result,
> whereas the %Lg format code gives something wrong.
Ah - sorry - I see now what you were trying to do.
> Can you try to check this by doing something like:
>
> - do some set of calculations using np.longdouble in Numpy
> (that requires the extra accuracy)
>
> - at the end, cast the result back to double
In [1]: import numpy as np
In [2]: res = np.longdouble(2)**64
In [6]: res / 2**32
Out[6]: 4294967296.0
In [7]: (res-1) / 2**32
Out[7]: 8589934591.9999999998
In [8]: np.float((res-1) / 2**32)
Out[8]: 4294967296.0
In [9]: np.float((res) / 2**32)
Out[9]: 4294967296.0
Thanks,
Matthew
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