Parallelizing a for loop using openmp & replacing push_back

Question

I\'d like to parallelize the following piece of code but am new to openmp and creating parallel code.

std::vector good_matches;
for (int i = 0;         


        

Answer 1

One possibility may be to use private vectors for each thread and combine them in the end:

#include

#include
#include
#include
#include

using namespace std;

int main()
{
  vector global_vector;  
  vector< vector > buffers;

  #pragma omp parallel
  {
    auto nthreads = omp_get_num_threads();
    auto id = omp_get_thread_num();
    //
    // Correctly set the number of buffers
    //
  #pragma omp single
    {
      buffers.resize( nthreads );
    }
    //
    // Each thread works on its chunk
    // If order is important maintain schedule static
    //
  #pragma omp for schedule(static)
    for(size_t ii = 0; ii < 100; ++ii) {      
      if( ii % 2 != 0 ) { // Any other condition will do
          buffers[id].push_back(ii);
      }
    }
    //
    // Combine buffers together
    //
    #pragma omp single
    {
      for( auto & buffer : buffers) {
        move(buffer.begin(),buffer.end(),back_inserter(global_vector));
      }
    }
  }
  //
  // Print the result
  //
  for( auto & x : global_vector) {
    cout << x << endl;
  }    
  return 0;
}

The actual speed-up depends only on the amount of work done inside each loop.