115. Example using MPI_SCAN

The example in this section uses an intracommunicator.
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 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++; 
    } 
}