gistfile1.f90

    subroutine step(positions, velocities)
        real, dimension(p,2), intent(inout) :: positions, velocities

        ! verify lengths (? unnecessary?)
        if (size(positions, 1) .ne. size(velocities, 1)) then
            print *, "The sizes of two arrays are not the same"
        end if

        ! move the particles to the new position 
        do i = 1, size(positions, 1)
            positions(i, 1) = positions(i, 1) + velocities(i, 1)
            positions(i, 2) = positions(i, 2) + velocities(i, 2)
        end do

    end subroutine step