lnmatrxc.h

ARPACK is a collection of Fortran77 subroutines designed to solve large scale eigenvalue problems.

/*
   ARPACK++ v1.0 8/1/1997
   c++ interface to ARPACK code.

   MODULE LNMatrxC.h
   Function template for the stiffness matrix obtained from
   the finite element discretization of the 1-dimensional
   convection diffusion operator d^2u/dx^2 + rho*(du/dx) on
   the interval [0,1] with zero Dirichlet boundary condition
   using linear elements.

   ARPACK Authors
      Richard Lehoucq
      Danny Sorensen
      Chao Yang
      Dept. of Computational & Applied Mathematics
      Rice University
      Houston, Texas
*/

#ifndef LNMATRXC_H
#define LNMATRXC_H

template<class FLOAT, class INT>
void StiffnessMatrix(INT n, FLOAT rho, INT& nnz,
                     FLOAT* &A, INT* &irow, INT* &pcol)
{

  INT   i, j;
  FLOAT dd, dl, du, s, h;

  // Defining constants.

  const FLOAT one = 1.0;
  const FLOAT two = 2.0;

  h  = one/FLOAT(n+1);
  s  = rho/two;
  dd = two/h;
  dl = -one/h - s;
  du = -one/h + s;

  // Defining the number of nonzero matrix elements.

  nnz = 3*n-2;

  // Creating output vectors.

  A    = new FLOAT[nnz];
  irow = new INT[nnz];
  pcol = new INT[n+1];

  // Filling A, irow and pcol.

  pcol[0] = 0;
  j = 0;
  for (i=0; i!=n; i++) {
    if (i != 0) {
      irow[j] = i-1;
      A[j++]  = du;
    }
    irow[j] = i;
    A[j++]  = dd;
    if (i != (n-1)) {
      irow[j] = i+1;
      A[j++]  = dl;
    }
    pcol[i+1] = j;
  }

} //  StiffnessMatrix.

#endif // LNMATRXC_H