mpf_class to double

Marc Glisse marc.glisse at
Sat Sep 4 17:50:08 CEST 2010

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.

Marc Glisse

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