search.f90

!! 
!! f2py -c -m search search.f90
!! 


subroutine find_first(needle, haystack, haystack_length, index)
    !!
    !! Find the first index of `needle` in `haystack`.
    !!
    implicit none
    integer, intent(in) :: needle
    integer, intent(in) :: haystack_length
    integer, intent(in), dimension(haystack_length) :: haystack
!f2py intent(inplace) haystack
    integer, intent(out) :: index
    integer :: k

    index = -1
    do k = 1, haystack_length
        if (haystack(k)==needle) then
            index = k - 1
            exit
        endif
    enddo
end

@hech92 Yeah Python knows this information, just Fortran doesn't, so Python can fill it in automatically before calling Fortran. And because it's always the same logic, it can be in the wrapper code so you don't have to do it.