openMP reduction and thread number control -

i use openmp as:

#pragma omp parallel reduction(+:average_stroke_width) for(int = 0; < gb_name.size(); ++i) {...} 

i know can use :

#pragma omp parallel num_threads(thread) for(int index = 0; index < gb_name.size(); ++index){...} 

how can control thread number when use reduction?

how can control thread number when use reduction?

both clauses can used togehter:

#pragma omp parallel reduction(+:average_stroke_width) num_threads(thread) for(int = 0; < gb_name.size(); ++i) {...} 

note reduction involves threads, cannot have parallel loop 8 threads , perform reduction 4 threads only. reduction combines local values in threads , therefore of them need participate.