C and Fortran: the right balance

I'm a fan of picking the right programming language for the job, even if it means mixing languages in a project.  Sometimes this can lead to pain, especially if I'm using relatively new language features that haven't had enough code coverage to be fully debugged.  However, getting the right balance between languages can make one's code concise and elegant.

This weekend, I've been working on rewriting a parallel matrix factorization.  I had written most of it in (modern) Fortran, but that made it tricky to interface with C code written by collaborators and others in our research group.  However, C's lack of 2-D array notation makes it awkward to write matrix operations.  Thankfully the factorization algorithm splits naturally into a series of operations on submatrices.  I implemented each of those "local" factorization steps in Fortran, using the ISO_C_BINDING module so that I can call them from C without needing to guess the Fortran compiler's name mangling convention, or needing to make everything a pointer (by default, Fortran calls everything by reference).  C code sequences those calls to local factorization steps and does the necessary parallel synchronization.  The resulting C is significantly shorter than the Fortran version I had written before, but I still get the benefits of Fortran's concise notation for matrix operations.