How to declare a variable mid-routine in Fortran

Question

I would like to create an array with a dimension based on the number of elements meeting a certain condition in another array. This would require that I initialize an array

Answer 1

You need to use an allocatable array (see this article for more on it). This would change your routine to

subroutine example(input_array,output_array)

  real,intent(in) :: input_array(50) ! passed array of known dimension
  real, intent(out), allocatable :: output_array(:)
  integer :: element_count, i

  element_count = 0
  do i=1,50
    if (some_array.gt.0) element_count = element_count+1
  enddo

  allocate(output_array(element_count))

end subroutine

Note that the intents may not be necessary, but are probably good practice. If you don't want to call a second array, it is possible to create a reallocate subroutine; though this would require the array to already be declared as allocatable.