How to write a program, which can be run on 1 and multiple processors?

[Yixun Liu]

Hi,
I want to make my code run on 1 or multiple processors. The code, which
can run on multiple processors is like the following,

MatCreate(PETSC_COMM_WORLD, &A);
   MatSetSizes(A, 3*numOfVerticesOfOneProcessor,
3*numOfVerticesOfOneProcessor, systemSize, systemSize);
   MatSetFromOptions(A);
   MatMPIAIJSetPreallocation(A, 50, PETSC_NULL, 50, PETSC_NULL);

However if I want to run on 1 processor I have to change the last code to:
MatSeqAIJSetPreallocation(A,1000,PETSC_NULL);

How to avoid changing code?

Thanks.

Yixun