array_reduction.f90

program array_reduction

  use omp_lib
  
  implicit none

  integer, parameter :: n = 20
  integer, parameter :: m = 1000000
  integer:: i,j
  integer, dimension(n) :: array_to_reduce
  integer, dimension(m) :: array_to_read

  array_to_reduce = 0
  
  do i = 1,m
     array_to_read(i) = 5
  end do

  ! This loop has a data race because multiple iterations are writing into
  ! array_to_reduce with the same value of j. It can be resolved by adding
  ! a reduction(+:array_to_reduce) clause to omp parallel do.
  !$omp parallel do default(shared) private(i,j) schedule(static)
  do i = 1,m
     j = mod(i,n) + 1
     array_to_reduce(j) = array_to_reduce(j) + array_to_read(i)
  end do
  !$omp end parallel do

  write(*,*) 'I am running with this many threads:', omp_get_max_threads()
  do i = 1,n
     write(*,*) array_to_reduce(i)
  end do
  
end program array_reduction