Data Parallelism - Java 8 Lambdas - page 100

for for ( int int i = 0 ; i < values . length ; i ++) {

values [ i ] = i ;

}
return

return values ;

}

We can use the parallelSetAll method in order to do this easily in parallel. An example of
this code is shown in Example 6-8 . We provide an array to operate on and a lambda expres-
sion, which calculates the value given the index. In our example they are the same value.
One thing to note about these methods is that they alter the array that is passed into the oper-
ation, rather than creating a new copy.

Example 6-8. Initializing an array using a parallel array operation
public

public static static double double [] parallelInitialize ( int int size ) {

double double [] values = new double [ size ];

Arrays . parallelSetAll ( values , i -> i );
return

new double

return values ;

}

The parallelPrefix operation, on the other hand, is much more useful for performing
accumulation-type calculations over time series of data. It mutates an array, replacing each
element with the sum of that element and its predecessors. I use the term “sum” loosely—it
doesn't need to be addition; it could be any BinaryOperator .

An example operation that can be calculated by prefix sums is a simple moving average.
This takes a rolling window over a time series and produces an average for each instance of
that window. For example, if our series of input data is 0, 1, 2, 3, 4, 3.5 , then the
simple moving average of size 3 is 1, 2, 3, 3.5 . Example 6-9 shows how we can use a
prefix sum in order to calculate a moving average.

Example 6-9. Calculating a simple moving average
public public static static double double [] simpleMovingAverage ( double double [] values , int int n ) {

double double [] sums = Arrays . copyOf ( values , values . length );

Arrays . parallelPrefix ( sums , Double: : sum );
int

int start = n - 1 ;
return return IntStream . range ( start , sums . length )

. mapToDouble ( i -> {
double double prefix = i == start ? 0 : sums [ i - n ];

return return ( sums [ i ] - prefix ) / n ;

})
. toArray ();

}