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Partitioning of Cartesian structures

MPI_CART_SUB(comm, remain_dims, newcomm)
[ IN comm] communicator with cartesian structure (handle)
[ IN remain_dims] the ith entry of remain_dims specifies whether the
ith dimension is kept in the subgrid ( true) or is dropped ( false) (logical vector)
[ OUT newcomm] communicator containing the subgrid that includes the calling process (handle)

int MPI_Cart_sub(MPI_Comm comm, int *remain_dims, MPI_Comm *newcomm)

MPI_CART_SUB(COMM, REMAIN_DIMS, NEWCOMM, IERROR)
INTEGER COMM, NEWCOMM, IERROR
LOGICAL REMAIN_DIMS(*)

If a cartesian topology has been created with MPI_CART_CREATE, the function
MPI_CART_SUB can be used to partition the communicator group into subgroups that form lower-dimensional cartesian subgrids, and to build for each subgroup a communicator with the associated subgrid cartesian topology. (This function is closely related to MPI_COMM_SPLIT.)

  
 
 
Assume that  MPI_CART_CREATE(..., comm) has defined a 

 grid. Let  remain_dims = (true, false, true). 
Then a call to, 
 
MPI_CART_SUB(comm, remain_dims, comm_new), 
 
will create three communicators each with eight processes
 
in a 
 cartesian
 
topology.  If  remain_dims = (false, false, true) then the call to
 
 MPI_CART_SUB(comm, remain_dims, comm_new)
 
will create six non-overlapping
 
communicators, each with four processes,
 
in a one-dimensional cartesian topology.