Intel Threading Building Blocks

ParallelSum

Example 11-19 sums the values in an array.

Example 11-19. ParallelSum

#include "tbb/parallel_reduce.h"
#include "tbb/blocked_range.h"

using namespace tbb;

struct Sum {
    float value;
    Sum() : value(0) {}
    Sum( Sum& s, split ) {value = 0;}
    void operator()( const blocked_range<float*>& range ) {
        float temp = value;
        for( float* a=range.begin(); a!=range.end(); ++a ) {
            temp += *a;
        }
        value = temp;
    }
    void join( Sum& rhs ) {value += rhs.value;}
};

float ParallelSum( float array[], size_t n ) {
    Sum total;
    parallel_reduce( blocked_range<float*>( array, array+n, 1000 ),
                     total );
    return total.value;
}

This example is easily converted to do a reduction for any associative operation op as follows:

1. Replace occurrences of 0 with the identity element for op.

2. Replace occurrences of += with op= or its logical equivalent.

3. Change the name Sum to something more appropriate for op.

The operation is allowed to be noncommutative. For example, op could be matrix multiplication.