I have some doubts about the following function that I have written in order to compute all the prime numbers in a specific interval. The resulting vector is not ordered but this is not a problem. I have some doubts about the consistency of the result between different rank, in particular about the casting part between ranks.

std::vector<unsigned> get_prime_numbers(unsigned a, unsigned b){

    int rank, size;

    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
    MPI_Comm_size(MPI_COMM_WORLD, &size);

    std::vector<unsigned> local_result;
    for(size_t i = a + rank; i <= b; i++){
        if(is_prime(i))
            local_result.push_back(i);
    }
    
    std::vector<unsigned> global_result;
    for(int r = 0; r < size; r++){
        if(rank == r){
            global_result.insert(global_result.end(), local_result.cbegin(), local_result.cend());
            unsigned n = local_result.size();
            MPI_Bcast(&n, 1, MPI_UNSIGNED, r, MPI_COMM_WORLD);
            MPI_Bcast(local_result.data(), n, MPI_UNSIGNED, r, MPI_COMM_WORLD);
        }
        else{
            unsigned n;
            MPI_Bcast(&n, 1, MPI_UNSIGNED, r, MPI_COMM_WORLD);
            std::vector<unsigned> rec(n);
            MPI_Bcast(local_result.data(), n, MPI_UNSIGNED, r, MPI_COMM_WORLD);
            global_result.insert(global_result.end(), rec.cbegin(), rec.cend());
        }
    }

    return global_result;

}

The function

is_prime(unsigned i) 

is an auxiliary function already implemented.

I think that the casting part is correct but I’m not sure