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Example using MPI_SCAN

  
 
 
This example uses a user-defined operation to produce a  segmented 
scan.  A segmented scan takes, as input, a set of values and a set of logicals, 
and the logicals delineate the various segments of the scan. 
For example: 

 
The operator that produces this effect is, 
 

 
where, 
 

 
Note that this is a non-commutative operator. 
C code that implements it is given below. 
 
 
typedef struct { 
    double val; 
    int log; 
} SegScanPair; 
 
/* the user-defined function 
 */ 
void segScan( SegScanPair *in, SegScanPair *inout, int *len, 
      MPI_Datatype *dptr ) 
{ 
    int i; 
    SegScanPair c; 
 
for (i=0; i< *len; ++i) { 
        if ( in->log == inout->log ) 
            c.val = in->val + inout->val; 
        else 
            c.val = inout->val; 
        c.log = inout->log; 
        *inout = c; 
        in++; inout++; 
    } 
} 
 
Note that the  inout argument to the user-defined function
 
corresponds to the right-hand operand of the operator.  When using
 
this operator, we must be careful to specify that it is non-commutative,
 
as in the following.
 
 

 
int i,base; 
    SeqScanPair  a, answer; 
    MPI_Op       myOp; 
    MPI_Datatype type[2] = {MPI_DOUBLE, MPI_INT}; 
    MPI_Aint     disp[2]; 
    int          blocklen[2] = { 1, 1}; 
    MPI_Datatype sspair; 
 
/* explain to MPI how type SegScanPair is defined 
     */ 
    MPI_Address( a, disp); 
    MPI_Address( a.log, disp+1); 
    base = disp[0]; 
    for (i=0; i<2; ++i) disp[i] -= base; 
    MPI_Type_struct( 2, blocklen, disp, type, &sspair ); 
    MPI_Type_commit( &sspair ); 
    /* create the segmented-scan user-op 
     */ 
    MPI_Op_create( segScan, False, &myOp ); 
    ... 
    MPI_Scan( a, answer, 1, sspair, myOp, root, comm );