Concatenate strings in a macro using gfortran

Question

The C preprocessor macro for concatenation ( ## ) does not seem to work on a Mac using gfortran. Using other Fortran compilers on other systems works so I am looking for a workaround for gfortran. I have to use the ## to create many variables so I can't do without them.

Example code:

#define CONCAT(x,y) x##y
program main
   integer, parameter:: CONCAT(ID,2) = 3
   print*,"Hello", ID_2
end program main

Compilation error with gfortran on MAC

gfortran m.F90 -o m
m.F90:5.23:
integer, parameter:: ID##2 = 3
                       1
Error: PARAMETER at (1) is missing an initializer

Answer 1

## doesn't work in gfortran (any OS, not just Mac) because it runs CPP in the traditional mode .

According to this thread the gfortran mailing list the correct operator in the traditional mode is x/**/y , so you must distinguish between different compilers:

#ifdef __GFORTRAN__
#define CONCAT(x,y) x/**/y
#else
#define CONCAT(x,y) x ## y
#endif

Others ( http://c-faq.com/cpp/oldpaste.html ) use this form, which behaves better when a macro passed to the CONCAT (via Concatenating an expanded macro and a word using the Fortran preprocessor ):

#ifdef __GFORTRAN__
#define PASTE(a) a
#define CONCAT(a,b) PASTE(a)b
#else
#define PASTE(a) a ## b
#define CONCAT(a,b) PASTE(a,b)
#endif

The indirect formulation helps to expand the passed macro before the strings are concatenated (it is too late after).