# MPI_Group_difference

## NAME

MPI_Group_difference − Makes a group from the difference of two groups.

## SYNTAX

## C Syntax

#include <mpi.h>
int MPI_Group_difference(MPI_Group group1, MPI_Group group2,

|  | MPI_Group *newgroup)

## Fortran Syntax

INCLUDE ’mpif.h’
MPI_GROUP_DIFFERENCE(GROUP1, GROUP2, NEWGROUP, IERROR)

| INTEGER |  |  | GROUP1, GROUP2, NEWGROUP, IERROR

## C++ Syntax

#include <mpi.h>
static Group Group::Difference(const Group& group1, const Group& group2)

## INPUT PARAMETERS

| group1 |  | First group (handle). |
| group2 |  | Second group (handle). |

## OUTPUT PARAMETERS

| newgroup |  | Difference group (handle). |
| IERROR |  | Fortran only: Error status (integer). |

## DESCRIPTION

The set-like operations are defined as follows:

| o |  | union -- All elements of the first group (group1), followed by all elements of second group (group2) that are not in the first group
| o |  | intersect -- all elements of the first group that are also in the second group, ordered as in first group
| o |  | difference -- all elements of the first group that are not in the second group, ordered as in the first group

Note that for these operations the order of processes in the output group is determined primarily by order in the first group (if possible) and then, if necessary, by order in the second group. Neither union nor intersection are commutative, but both are associative.

The new group can be empty, that is, equal to MPI_GROUP_EMPTY.

## ERRORS

Almost all MPI routines return an error value; C routines as the value of the function and Fortran routines in the last argument. C++ functions do not return errors. If the default error handler is set to MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception mechanism will be used to throw an MPI:Exception object.

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_Group_free

MPI API Index