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The routine MPI_Cart_shift finds the neighbors in each direction of the new communicator.
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dir = 0; // in C 0 for columns, 1 for rows
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// in Fortran, it's 1 and 2
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disp = 1; // specifies first neighbor to the right and left
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ierr = MPI_Cart_shift (comm2d, dir, disp, &nbrbottom,
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&nbrtop):
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This returns the process numbers (ranks) for a communication of the bottom and top neighbors.
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If a process in a non-periodic mesh is on the border and has no neighbor, then the value MPI_PROCNULL is returned. This process value can be used in a send/recv, but it will have no effect.
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