ColMatrixVectorMultiply(int n, double *a, double *b, double *x,MPI_Comm comm) 
{ 
  int i, j; 
  int nlocal; 
  double *px; 
  double *fx; 
  int npes, myrank; 
  MPI_Status status; 

  /* Get identity and size information from the communicator */ 
  MPI_Comm_size(comm, &npes); 
  MPI_Comm_rank(comm, &myrank); 

  nlocal = n/npes; 

  /* Allocate memory for arrays storing intermediate results. */ 
  px = (double *)malloc(n*sizeof(double)); 
  fx = (double *)malloc(n*sizeof(double)); 

  /* Compute the partial-dot products that correspond to the local columns of A.*/ 
  for (i=0; i<n; i++) { 
    px[i] = 0.0; 
    for (j=0; j<nlocal; j++) 
      px[i] += a[i*nlocal+j]*b[j]; 
  } 

  /* Sum-up the results by performing an element-wise reduction operation */ 
  MPI_Reduce(px, fx, n, MPI_DOUBLE, MPI_SUM, 0, comm); 

  /* Redistribute fx in a fashion similar to that of vector b */ 
  MPI_Scatter(fx, nlocal, MPI_DOUBLE, x, nlocal, MPI_DOUBLE, 0,comm); 

  free(px); free(fx); 
}