Thanks, This was a stupid Boost beginner question Boost is great After calculating, with Maxima, the symbolic equations of some analog circuits, I am doing numeric simulations based on recursive convolution involving for example exponentiation of matrices or inversions Depending on components values the "long double" are sometime not enough Regards, Jacques ________________________________ De : Boost-users de la part de John Maddock via Boost-users Envoyé : vendredi 26 novembre 2021 20:20 À : Jacques Mequin via Boost-users Cc : John Maddock Objet : Re: [Boost-users] assigning a "float128" variable with a regular "double" variable On 26/11/2021 17:50, Jacques Mequin via Boost-users wrote:

I am a fresh new user ( I need to extend some EIGEN library feature by using your float128 )

How to assign a "float128" variable with a regular "double" variable ?

With my source code in attachement

double double_pi = 3.141592653589793 ; float128 float128_pi = 3.141592653589793Q ;

produces the output that looks fine to me

double_pi = 3.141592653589793e+00

float128_pi = 3.141592653589793000000000000000000e+00

but thestatement float128_pi = double_pi ; produces

float128_pi = 3.141592653589793115997963468544185e+00

How to get 3.141592653589793000000000000000000e+00 also from an assigment ???

You can't: remember your double has 53-bit precision, 3.141592653589793 is it's decimal value ROUNDED to 16 decimal places (note that you would have to print more digits than that to be able to round-trip to and from decimal), converting to a float128_t gives you EXACTLY the same binary value, but you're code is now printing out more decimal places, so you're seeing it's "true" decimal value, rather than the rounded value you had before from the double output. Hope that makes sense, John.