subroutine euler ( dydt, tspan, y0, n, m, t, y ) !*****************************************************************************80 ! !! euler() approximates the solution to an ODE using Euler's method. ! ! Licensing: ! ! This code is distributed under the MIT license. ! ! Modified: ! ! 25 April 2020 ! ! Author: ! ! John Burkardt ! ! Input: ! ! external dydt: a subroutine that evaluates the right ! hand side of the ODE. ! ! real ( kind = rk ) tspan(2): contains the initial and final times. ! ! real ( kind = rk ) y0(m): a column vector containing the initial condition. ! ! integer n: the number of steps to take. ! ! integer m: the number of variables. ! ! Output: ! ! real ( kind = rk ) t(n+1), y(n+1,m): the times and solution values. ! implicit none integer, parameter :: rk = kind ( 1.0D+00 ) integer m integer n real ( kind = rk ) dt real ( kind = rk ) dy(m) external dydt integer i real ( kind = rk ) t(n+1) real ( kind = rk ) tfirst real ( kind = rk ) tlast real ( kind = rk ) tspan(2) real ( kind = rk ) y(n+1,m) real ( kind = rk ) y0(m) tfirst = tspan(1) tlast = tspan(2) dt = ( tlast - tfirst ) / real ( n, kind = rk ) t(1) = tspan(1) y(1,1:m) = y0(1:m) do i = 1, n t(i+1) = t(i) + dt call dydt ( t(i), y(i,1:m), dy ) y(i+1,1:m) = y(i,1:m) + dt * dy(1:m) end do return end