# Name

`MPI_Graph_create` - Makes a new communicator to which topology
information has been attached.

# Syntax

## C Syntax

```c
#include <mpi.h>

int MPI_Graph_create(MPI_Comm comm_old, int nnodes, const int index[],
    const int edges[], int reorder, MPI_Comm *comm_graph)
```

## Fortran Syntax

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

MPI_GRAPH_CREATE(COMM_OLD, NNODES, INDEX, EDGES, REORDER,
    	COMM_GRAPH, IERROR)
    INTEGER	COMM_OLD, NNODES, INDEX(*), EDGES(*)
    INTEGER	COMM_GRAPH, IERROR
    LOGICAL   REORDER
```

## Fortran 2008 Syntax

```fortran
USE mpi_f08

MPI_Graph_create(comm_old, nnodes, index, edges, reorder, comm_graph,
    	ierror)
    TYPE(MPI_Comm), INTENT(IN) :: comm_old
    INTEGER, INTENT(IN) :: nnodes, index(nnodes), edges(*)
    LOGICAL, INTENT(IN) :: reorder
    TYPE(MPI_Comm), INTENT(OUT) :: comm_graph
    INTEGER, OPTIONAL, INTENT(OUT) :: ierror
```


# Input Parameters

* `comm_old` : Input communicator without topology (handle).
* `nnodes` : Number of nodes in graph (integer).
* `index` : Array of integers describing node degrees (see below).
* `edges` : Array of integers describing graph edges (see below).
* `reorder` : Ranking may be reordered (true) or not (false) (logical).

# Output Parameters

* `comm_graph` : Communicator with graph topology added (handle).
* `IERROR` : Fortran only: Error status (integer).

# Description

`MPI_Graph_create` returns a handle to a new communicator to which the
graph topology information is attached. If `reorder` = false then the rank
of each process in the new group is identical to its rank in the old
group. Otherwise, the function may `reorder` the processes. If the size,
`nnodes`, of the graph is smaller than the size of the group of `comm_old`,
then some processes are returned `MPI_COMM_NULL`, in analogy to
`MPI_Cart_create` and `MPI_Comm_split`. The call is erroneous if it
specifies a graph that is larger than the group size of the input
communicator.

The three parameters `nnodes`, `index`, and `edges` define the graph
structure. `nnodes` is the number of nodes of the graph. The nodes are
numbered from 0 to `nnodes`-1. The ith entry of array `index` stores the
total number of neighbors of the first i graph nodes. The lists of
neighbors of nodes 0, 1, ..., `nnodes`-1 are stored in consecutive
locations in array `edges`. The array `edges` is a flattened representation
of the edge lists. The total number of entries in `index` is `nnodes` and
the total number of entries in `edges` is equal to the number of graph
`edges`.

The definitions of the arguments `nnodes`, `index`, and `edges` are
illustrated with the following simple example.

Example: Assume there are four processes 0, 1, 2, 3 with the
following adjacency matrix:

| Process | Neighbors |
| ------- | --------- |
|    0	  |   1, 3    |
|    1	  |	0     |
|    2	  |	3     |
|    3	  |   0, 2    |

Then, the input arguments are:
* `nodes` = 4
* `index`  = 2, 3, 4, 6
* `edges`  = 1, 3, 0, 3, 0, 2

Thus, in C, `index[0]` is the degree of `node` zero, and `index[i]` -
`index[i-1]` is the degree of `node` i, i=1, . . . , nnodes-1; the list of
neighbors of node zero is stored in `edges[j]`, for 0 <= j <=
`index[0] - 1` and the list of neighbors of `node` i, i > 0 , is stored
in `edges[j]`, `index[i-1]` <= j <= `index[i] - 1`.

In Fortran, `index(1)` is the degree of `node` zero, and `index(i+1)` -
`index(i)` is the degree of `node` i, i=1, . . . , nnodes-1; the list of
neighbors of `node` zero is stored in `edges(j)`, for 1 <= j <= `index(1)`
and the list of neighbors of `node` i, i > 0, is stored in `edges(j)`,
`index(i) + 1` <= j <= `index(i + 1)`.

# 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_Graph_get`(3)](MPI_Graph_get.html)
[`MPI_Graphdims_get`(3)](MPI_Graphdims_get.html)