In our case, since we are summing the rectangle contributions from every process we define $MPI\_Op = MPI\_SUM$. If we have an array or matrix we can search for the largest og smallest element by sending either $MPI\_MAX$ or $MPI\_MIN$. If we want the location as well (which array element) we simply transfer $MPI\_MAXLOC$ or $MPI\_MINOC$. If we want the product we write $MPI\_PROD$.

$MPI\_Allreduce$ is defined as

MPI_Allreduce( void *senddata, void* resultdata, int count, 
          MPI_Datatype datatype, MPI_Op, MPI_Comm comm)