Advanced Example

An example of a more advanced associative operation is to find the index where Foo(i) is minimized. A serial version might look like this:

long SerialMinIndexFoo( const float a[], size_t n ) {
    float value_of_min = FLT_MAX;        // FLT_MAX from <climits>
    long index_of_min = -1;
    for( size_t i=0; i<n; ++i ) {
        float value = Foo(a[i]);
        if( value<value_of_min ) {
            value_of_min = value;
            index_of_min = i;
        }
    }
    return index_of_min;
}

The loop works by keeping track of the minimum value found so far, and the index of this value. This is the only information carried between loop iterations. To convert the loop to use parallel_reduce, the function object must keep track of the carried information, and how to merge this information when iterations are spread across multiple threads. Also, the function object must record a pointer to a to provide context.

The following code shows the complete function object.

class MinIndexFoo {
    const float *const my_a;
public:
    float value_of_min;
    long index_of_min;
    void operator()( const blocked_range<size_t>& r ) {
        const float *a = my_a;
        for( size_t i=r.begin(); i!=r.end(); ++i ) {
           float value = Foo(a[i]);
           if( value<value_of_min ) {
               value_of_min = value;
               index_of_min = i;
           }
        }
    }


    MinIndexFoo( MinIndexFoo& x, split ) :
        my_a(x.my_a),
        value_of_min(FLT_MAX),    // FLT_MAX from <climits>
        index_of_min(-1)
   {}


    void join( const SumFoo& y ) {
        if( y.value_of_min<value_of_min ) {
            value_of_min = y.value_of_min;
            index_of_min = y.index_of_min;
        }
    }


    MinIndexFoo( const float a[] ) :
        my_a(a),
        value_of_min(FLT_MAX),    // FLT_MAX from <climits>
        index_of_min(-1),
    {}
};

Now SerialMinIndex can be rewritten using parallel_reduce as shown below:

long ParallelMinIndexFoo( float a[], size_t n ) {
    MinIndexFoo mif(a);
    parallel_reduce(blocked_range<size_t>(0,n), mif );


 return mif.index_of_min;
}