nclude #include double generate_people(int i, int j) { return (2 * i + 3 * j); // some dummy return value } int main() { int num_threads = 4; omp_set_num_threads(num_threads); int num = 10; int people = 0; int apples = 5000; #pragma omp parallel for reduction(+:people) reduction(-:apples) for (int i = 0; i < num; i++) { for (int j = i+1; j < num; j++) { int ppl = generate_people(i, j); people += ppl; apples -= ppl; } } printf("people = %d ", people); printf("apples = %d", apples); } /* You can see what the use of the reduction clause is by making people shared. You have noticed the difference in the outputs. Why does this happen? Race conditions take place when multiple threads read and write a variable simultaneously. They give us random results that depend on the order the threads access the variable. Reduction is a mechanism to control this access and helps avoid the race condition. Reduction takes care of making a private copy of the variable for each thread. After the parallel region, the reduction operation is applied to private variables and the result is aggregated to the shared variable. */