[petsc-users] Why PETSc needs PetscScalar datatype?

[Satish Balay]

You could try the following fix.

diff --git a/include/petsc/finclude/petscsys.h b/include/petsc/finclude/petscsys.h
index 4ca4ebb8ba..cffbafd475 100644
--- a/include/petsc/finclude/petscsys.h
+++ b/include/petsc/finclude/petscsys.h
@@ -150,6 +150,7 @@
 !
 !     Macro for templating between real and complex
 !
+#define PetscComplex PetscFortranComplex
 #if defined(PETSC_USE_COMPLEX)
 #define PetscScalar PetscFortranComplex
 !


Satish

On Sat, 4 Aug 2018, Satish Balay wrote:

> For some reason it is PetscFortranComplex [on fortran side]
> 
> I'm not sure I understand your logic for using PetscComplex and not PetscScalar.
> 
> We recommend using PetscScalar. You can choose not to follow our
> recommendation.
> 
> But we don't say 'PetscScalar' is the wrong thing for users [as you
> appear to infer]
> 
> Satish
> 
> On Sat, 4 Aug 2018, Youjun Hu wrote:
> 
> > I just tried to change PetscScalar to the specific datatype PetscComplex in
> > my Fortran code, but the compiler raised the following error:
> > PetscComplex   my_coeff
> > Error: Unclassifiable statement at (1)
> > 
> > It seems that the compiler does not know this datatype.
> > 
> > My Petsc was configured with ./configure   --with-scalar-type=complex
> > --with-precision=double
> > 
> > Before the modification, the code was complied successfully.
> > Could someone help?
> > 
> > Thanks,
> > Youjun
> > 
> > On Sat, Aug 4, 2018 at 12:28 PM Smith, Barry F. <bsmith at mcs.anl.gov> wrote:
> > 
> > >
> > >
> > > > On Aug 4, 2018, at 1:11 PM, Youjun Hu <youjunhu at gmail.com> wrote:
> > > >
> > > > Thank Barry for the clarification.
> > > > In summary, application code developers should choose the specific
> > > datatypes (PetscReal and PetscComplex) rather than the more general
> > > datatype PetscScalar
> > > > because application codes usually have fixed datatype and the
> > > flexibility provided by PetscScalar is for PETSc library developers and
> > > usually not for application developers.
> > > > If an application developer uses PetscScalar to declare a variable that
> > > is intended by the developer to be complex and is assigned a complex value
> > > later in the code and then used in, e.g.,  MatSetValues, but it happens
> > > that petsc is configured with --with-scalar-type=real
> > > > then, it is not possible for this error to be detected at compiling
> > > time. If the developer specify PetscComplex, then this inconsistency can be
> > > seen by compilers and hopefully an error message can be raised during
> > > compiling.
> > > >
> > > > Conclusion: avoid using PetscScalar in application codes unless it is
> > > absolutely needed (which is not common). The example application codes
> > > provided in tutor folds of petsc often use PetscScalar, which is a little
> > > misleading and should better be revised to use concrete datatypes.
> > >
> > >     The reason the examples use PetscScalar is so that all the examples
> > > can be used to test both the real and complex versions of the libraries. If
> > > they used the specific types they could only be testing one or the other.
> > >
> > >     Note there is no harm in writing applications with PetscScalar it is
> > > just usually not needed.
> > >
> > >    Barry
> > >
> > > > Correct me if anybody finds this conclusion is not justified.
> > > >
> > > > Cheers,
> > > > Youjun
> > > >
> > > >
> > > >
> > > > On Sat, Aug 4, 2018 at 11:03 AM Smith, Barry F. <bsmith at mcs.anl.gov>
> > > wrote:
> > > >
> > > >   For a particular configuration (./configure --with-scalar-type= real
> > > or complex) the PETSc objects, such as vectors and matrices either contain
> > > real values or complex values. In the same running code it is not possible
> > > to have some vectors with real numbers and some vectors with complex
> > > numbers.
> > > >
> > > >   Depending on how you configure PETSc ./configure --with-scalar-type=
> > > real or complex the variable name PetscScalar becomes either PetscReal or
> > > PetscComplex. PETSc code is written with PetscScalar so we can reconfigure
> > > the same source code and have it work with real numbers or complex.
> > > >
> > > >    For users who just work with real numbers you can simply use
> > > PetscReal as your variable type. If working with just with complex numbers
> > > you can use PetscComplex. If you want to sometimes configure with real
> > > numbers and sometimes configure with complex (which is not common) you use
> > > PetscScalar.
> > > >
> > > >    Barry
> > > >
> > > >
> > > > > On Aug 4, 2018, at 11:39 AM, Youjun Hu <youjunhu at gmail.com> wrote:
> > > > >
> > > > > Hi all,
> > > > >
> > > > > I am just wondering why PETSc introduces PetscScalar datatype in
> > > addition to the PetscReal and PetscComplex.
> > > > > Could someone help clarify the rationale behind this design?
> > > > > I am primarily a Fortran guy. The additional datatype PetscScalar
> > > seems to only confuse me rather than provide me any flexibility.
> > > > > Cheers,
> > > > > Youjun Hu
> > > >
> > >
> > >
> > 
>