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- program main
- implicit none
- integer, parameter :: dp = 8
- real(dp), dimension(1:3) :: x
- integer, parameter :: nstep = 20000000
- real(dp) :: t = 0.0_dp
- real(dp) :: h = 1.0e-10_dp
- integer, parameter :: nb_loops = 21
- integer, parameter :: n = 3
- integer :: k
- integer :: time_begin
- integer :: time_end
- integer :: count_rate
- real(dp) :: time
- real(dp) :: min_time = 100.0
- do k = 1, nb_loops
- x = [ 8.5_dp, 3.1_dp, 1.2_dp ]
- call system_clock(time_begin, count_rate)
- call rk4sys(n, t, x, h, nstep)
- call system_clock(time_end, count_rate)
- time = real(time_end - time_begin, dp) / real(count_rate, dp)
- min_time = min(time, min_time)
- write (*,*) time, x(1)
- end do
- write (*,*) "Minimal Runtime:", min_time
- contains
- subroutine xpsys(x,f)
- real(dp), dimension(1:3), intent(in) :: x
- real(dp), dimension(1:3), intent(out) :: f
- f(1) = 10.0_dp * ( x(2) - x(1) )
- f(2) = 28.0_dp * x(1) - x(2) - x(1) * x(3)
- f(3) = x(1) * x(2) - (8.0_dp / 3.0_dp) * x(3)
- end subroutine xpsys
- subroutine rk4sys(n, t, x, h, nstep)
- integer, intent(in) :: n
- real(dp), intent(in) :: t
- real(dp), dimension(1:n), intent(inout) :: x
- real(dp), intent(in) :: h
- integer, intent(in) :: nstep
- ! Local variables
- real(dp) :: h2
- real(dp), dimension(1:n) :: y, f1, f2, f3, f4
- integer :: i, k
- h2 = 0.5_dp * h
- do k = 1, nstep
- call xpsys(x, f1)
- y = x + h2 * f1
- call xpsys(y, f2)
- y = x + h2 * f2
- call xpsys(y, f3)
- y = x + h * f3
- call xpsys(y, f4)
- x = x + h * (f1 + 2.0_dp * (f2 + f3) + f4) / 6.0_dp
- end do
- end subroutine rk4sys
- end program main
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