Sending a Column

double A[10][10]

if (!my_id)

{MPI_Send(&A[2][0], 10, MPI_DOUBLE, 1, 0, MPI_COMM_WORLD);}

else if (my_id==1)

{MPI_Recv(&A[2][0], 10, MPI_DOUBLE, 0, 0, MPI_COMM_WORLD, &status);}

But what if I want to send a column?

MPI_Datatype col_t;

MPI_Type_vector(10, 1, 10, MPI_DOUBLE, &col_t);

MPI_Type_commit(&col_t);

if (!my_id)

{MPI_Send(&A[0][2], 1, col_t, 1, 0, MPI_COMM_WORLD);}

else if (my_id==1)

{MPI_Recv(&A[0][2], 1, col_t, 0, 0, MPI_COMM_WORLD, &status);}

Note that since the address isn’t bound with the derived datatype we can use col_t

to send any column of any 10x10 matix of doubles!

Reuse is important since it’s fairly expensive to construct a derived datatype