# Name

`MPI_Cart_sub` - Partitions a communicator into subgroups, which form
lower-dimensional Cartesian subgrids.

# Syntax

## C Syntax

```c
#include <mpi.h>

int MPI_Cart_sub(MPI_Comm comm, const int remain_dims[], MPI_Comm *comm_new)
```

## Fortran Syntax

```fortran
USE MPI
! or the older form: INCLUDE 'mpif.h'

MPI_CART_SUB(COMM, REMAIN_DIMS, COMM_NEW, IERROR)
    INTEGER COMM, COMM_NEW, IERROR
    LOGICAL REMAIN_DIMS(*)
```

## Fortran 2008 Syntax

```fortran
USE mpi_f08

MPI_Cart_sub(comm, remain_dims, newcomm, ierror)
    TYPE(MPI_Comm), INTENT(IN) :: comm
    LOGICAL, INTENT(IN) :: remain_dims(*)
    TYPE(MPI_Comm), INTENT(OUT) :: newcomm
    INTEGER, OPTIONAL, INTENT(OUT) :: ierror
```

# Input Parameters

* `comm` : Communicator with Cartesian structure (handle).
* `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).

# Output Parameters

* `comm_new` : Communicator containing the subgrid that includes the calling
process (handle).
* `IERROR` : Fortran only: Error status (integer).

# Description

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`.)

Example: Assume that `MPI_Cart_create( ..., comm)` has defined a (2 x
3 x 4) 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 2 x 4
Cartesian topology. If `remain_dims = (false, false, true)` then the call
to `MPI_Cart_sub(comm, remain_dims, comm_new)` will create six
nonoverlapping communicators, each with four processes, in a
one-dimensional Cartesian topology.

# Errors

Almost all MPI routines return an error value; C routines as the value
of the function and Fortran routines in the last argument.
Before the error value is returned, the current MPI error handler is
called. By default, this error handler aborts the MPI job, except for
I/O function errors. The error handler may be changed with
`MPI_Comm_set_errhandler`; the predefined error handler `MPI_ERRORS_RETURN`
may be used to cause error values to be returned. Note that MPI does not
guarantee that an MPI program can continue past an error.

# See Also

[`MPI_Cart_create`(3)](MPI_Cart_create.html)
[`MPI_Comm_split`(3)](MPI_Comm_split.html)