Indexed

The Indexed constructor allows one to specify a noncontiguous data layout where displacements between successive blocks need not be equal. This allows one to gather arbitrary entries from an array and send them in one message, or receive one message and scatter the received entries into arbitrary locations in an array.

MPI_TYPE_INDEXED(count, array_of_blocklengths, array_of_displacements, 
                    oldtype, newtype)
   IN        count                       number of blocks
   IN        array_of_blocklengths       number of elements per block
   IN        array_of_displacements      displacement for each block, measured
                                          as number of elements
   IN        oldtype                     old datatype
   OUT       newtype                     new datatype

MPI_TYPE_INDEXED(COUNT, ARRAY_OF_BLOCKLENGTHS, ARRAY_OF_DISPLACEMENTS, OLDTYPE, NEWTYPE, IERROR)INTEGER COUNT, ARRAY_OF_BLOCKLENGTHS(*), ARRAY_OF_DISPLACEMENTS(*), OLDTYPE, NEWTYPE, IERROR

MPI_TYPE_INDEXED allows replication of an old datatype into a sequence of blocks (each block is a concatenation of the old datatype), where each block can contain a different number of copies of oldtype and have a different displacement. All block displacements are measured in units of the oldtype extent. The action of the Indexed constructor is represented schematically in Figure .

Figure: Datatype constructor MPI_TYPE_INDEXED.

and

B[j] = blocklength, j = 0,.......,count - 1

  The use of the MPI_TYPE_INDEXED function was illustrated in Example 3.1
on page 102; the function was used to transfer the upper triangular pert of a
square matrix.