Re: assigning a "float128" variable with a regular "double" variable
John Maddock via Boost-users <[email protected]>
| Newsgroups | gmane.comp.lib.boost.user |
|---|---|
| Message-ID | <[email protected]> |
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. -- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus _______________________________________________ Boost-users mailing list [email protected] https://lists.boost.org/mailman/listinfo.cgi/boost-users