mpf_class to double

[Andreas Puettmann]

  Thanks everyone for your rapid and enlightening answers!

I just integrated the arbitrary precision types into my code, and it 
really makes a difference!
I think I will also try the trick suggested by Tor Myklebust.

Great help guys!


On 09/04/2010 05:50 PM, Marc Glisse wrote:
> On Sat, 4 Sep 2010, Paul Zimmermann wrote:
>
>>       Andreas,
>>
>>> And what I would like to do is something like this (in some sort of
>>> pseudo code):
>>>
>>> void calc_sum (double *a, double *result, int l)
>>> {
>>>      // convert input values to high precision types... in whatever 
>>> way...
>>>      mpf_class mpa[l];
>
> You don't need to store them in an array, you can convert them on the 
> fly, always reusing the same mpf_class object, or you can let 
> mpresult+=a[i] do the conversion implicitly for you.
>
>>>      mpf_class mpresult = 0.0;
>>>
>>>      for(int i=0; i<l; i++)
>>>          mpa[i]= a[i];
>>>
>>>
>>>      // evaluate sum with mpf types
>>>      for(int i=0; i<l; i++)
>>>          mpresult += mpa[i];
>>>
>>>
>>>      // convert result back to double
>>>      *result = mpf_to_double( mpresult );
>>> }
>>>
>>>
>>> Right now this fails mainly because I don't know how to do the
>>> back-conversion from mpf to double.
>>
>> see the mpf_get_d() function.
>>
>>> But I would also like to know if this approach is intelligent at 
>>> all. Or
>>> would there be a nicer/better way to do this?
>>
>> this approach seems good to me. Don't forget to set the precision of 
>> the mpf
>> numbers to a large value (I don't know how to do this with the C++ 
>> interface).
>
> With the C++ interface, assuming you have a mpf_class f, you could 
> write f.get_mpf_t() and use the C functions on it, or directly use the 
> C++ versions, which have the same name without the mpf_ prefix, i.e. 
> f.get_d(), f.set_prec(n), etc.
>