The first 2 examples are not dividing the work between the threads, but having each of the threads repeat the full work, which is not poor OpenMP use, but wrong

The first 2 examples are not dividing the work between the threads, but having each of the threads repeat the full work, which is not poor OpenMP use, but wrong use. I would have also used the collapse directive and played a little bit with the schedule. Finally, looping in the inner loop through the first index is not a good idea not only when working with OpenMP.