"omp parallel for" does not work in "omp parallel"

Question

I expect to get the following output:

My rank is: 0 num is: 0
My rank is: 1 num is: 1
My rank is: 2 num is: 2
My rank is: 3 num is: 3

from the following code:

#pragma omp parallel
{
   int my_rank = omp_get_thread_num();

   #pragma omp parallel for num_threads(4)
   for(int i = 0; i < 4; i++){       
       printf("My rank is: %d num is: %d\n",my_rank, i);       
   }
}

But it gives following output:

My rank is: 0 num is: 0
My rank is: 0 num is: 1
My rank is: 0 num is: 2
My rank is: 0 num is: 3
My rank is: 2 num is: 0
My rank is: 2 num is: 1
My rank is: 2 num is: 2
My rank is: 2 num is: 3
My rank is: 3 num is: 0
My rank is: 3 num is: 1
My rank is: 3 num is: 2
My rank is: 3 num is: 3
My rank is: 1 num is: 0
My rank is: 1 num is: 1
My rank is: 1 num is: 2
My rank is: 1 num is: 3

What is the problem?

Answer 1

You should not repeat parallel, you are already inside a parallel block, so you only need pragma omp for for the loop, and each thread executing the parallel block will automatically take a chunk of the loop if you specify pragma omp for. If you want to specify the number of threads you can do pragma omp parallel num_threads(4) and then pragma omp for. In any case for such a simple piece of code you can just drop the entire outer block which seems unneeded.

Here's the correct version:

#pragma omp parallel num_threads(4)
{
  int my_rank = omp_get_thread_num();

  #pragma omp for
  for(int i = 0; i < 4; i++){       
      printf("My rank is: %d num is: %d\n", my_rank, i);       
  }
}

Or simply:

#pragma omp parallel for num_threads(4)
for(int i = 0; i < 4; i++){       
    printf("My rank is: %d num is: %d\n", omp_get_thread_num(), i);       
}