7.11.3. Example using MPI_SCAN

The example in this section uses an intra-communicator.

Example 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 noncommutative 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 noncommutative, as in the following.

int i,base; 
SegScanPair  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_Get_address(&a, disp); 
MPI_Get_address(&a.log, disp+1); 
base = disp[0]; 
for (i=0; i<2; ++i) disp[i] -= base; 
MPI_Type_create_struct(2, blocklen, disp, type, &sspair); 
MPI_Type_commit(&sspair); 
/* create the segmented-scan user-op 
 */ 
MPI_Op_create(segScan, 0, &myOp); 
... 
MPI_Scan(&a, &answer, 1, sspair, myOp, comm);