DOLFIN User Manual - FEniCS Project

DOLFIN User ManualHoffman, Jansson, Logg, Wells2.2.2 Writing the solverHaving compiled the variational problem (2.4) with FFC, it is now easy toimplement a solver for Poisson’s equation. We first discuss the implementationline by line and then present the complete program. The source codefor this example is available in the directory src/demo/pde/poisson/ of theDOLFIN source tree.At the beginning of our C++ program, which we write in a text file namedmain.cpp, we must first include the header file dolfin.h, which gives ourprogram access to the DOLFIN class library. In addition, we include theheader file Poisson.h generated by the form compiler. Since all classes inthe DOLFIN class library are defined within the namespace dolfin, we alsospecify that we want to work within this namespace:#include #include ‘‘Poisson.h’’using namespace dolfin;Since we are writing a C++ program, we need to create a main function.You are free to organize your program any way you like, but in this simpleexample we just write our program inside the main function:int main(){// Write your program herereturn 0;}We now proceed to specify the right-hand side f of (2.1). This is done bydefining a new subclass of Function and overloading the eval() function toreturn the value f(x, y) = x sin(y):18