for reduction( :)s

"for reduction(:)" is a syntax used in parallel programming to specify a reduction operation on a set of values. It is commonly used in languages such as OpenMP and MPI to perform parallel reductions on arrays, lists, and other data structures.

The syntax "for reduction(oper: var)" specifies that the variable "var" will be used in a reduction operation, using the reduction operator "oper". The reduction operator can be any arithmetic or logical operator, such as +, *, &, |, etc.

For example, consider the following code snippet:

int sum = 0;
#pragma omp parallel for reduction(+:sum)
for (int i = 0; i < N; i++) {
    sum += array[i];
}

This code uses OpenMP to perform a parallel sum of an array "array" of size N. The "reduction(+:sum)" syntax specifies that each thread will maintain a private copy of the variable "sum", and at the end of the loop, all private copies will be combined using the "+" operator to produce the final sum.

The "for reduction(:)" syntax is a shorthand for specifying a reduction operation on all variables in a loop. For example, consider the following code:

int sum = 0, prod = 1;
#pragma omp parallel for reduction(+:sum) reduction(*:prod)
for (int i = 0; i < N; i++) {
    sum += array[i];
    prod *= array[i];
}

This code performs both a sum and a product of the array "array" in parallel, using the "reduction(+:sum)" and "reduction(*:prod)" syntaxes to specify the reduction operations.