Task: Animate a pendulum



C#: using System;
using System.Drawing;
using System.Windows.Forms;

class CSharpPendulum
{
    Form _form;
    Timer _timer;
    
    double _angle = Math.PI / 2, 
           _angleAccel, 
           _angleVelocity = 0, 
           _dt = 0.1;
    
    int _length = 50;

    [STAThread]
    static void Main()
    {
        var p = new CSharpPendulum();
    }

    public CSharpPendulum()
    {
        _form = new Form() { Text = "Pendulum", Width = 200, Height = 200 };
        _timer = new Timer() { Interval = 30 };

        _timer.Tick += delegate(object sender, EventArgs e)
        {
            int anchorX = (_form.Width / 2) - 12,
                anchorY = _form.Height / 4,
                ballX = anchorX + (int)(Math.Sin(_angle) * _length),
                ballY = anchorY + (int)(Math.Cos(_angle) * _length);

            _angleAccel = -9.81 / _length * Math.Sin(_angle);
            _angleVelocity += _angleAccel * _dt;
            _angle += _angleVelocity * _dt;
          
            Bitmap dblBuffer = new Bitmap(_form.Width, _form.Height);
            Graphics g = Graphics.FromImage(dblBuffer);
            Graphics f = Graphics.FromHwnd(_form.Handle);

            g.DrawLine(Pens.Black, new Point(anchorX, anchorY), new Point(ballX, ballY));
            g.FillEllipse(Brushes.Black, anchorX - 3, anchorY - 4, 7, 7);
            g.FillEllipse(Brushes.DarkGoldenrod, ballX - 7, ballY - 7, 14, 14);
            
            f.Clear(Color.White);
            f.DrawImage(dblBuffer, new Point(0, 0));    
        };

        _timer.Start();
        Application.Run(_form);
    }     
}








Fortran: 
program animated_pendulum
implicit none
double precision, parameter :: pi = 4.0D0*atan(1.0D0), l = 1.0D-1, dt = 1.0D-2, g = 9.8D0
integer :: io
double precision :: s_ang, c_ang, p_ang, n_ang

write(*,*) 'Enter starting angle (in degrees):'
do
  read(*,*,iostat=io) s_ang
  if(io.ne.0 .or. s_ang.lt.-90.0D0 .or. s_ang.gt.90.0D0) then
    write(*,*) 'Please enter an angle between 90 and -90 degrees:'
  else
    exit
  end if
end do
call execute_command_line('cls')

c_ang = s_ang*pi/180.0D0
p_ang = c_ang

call display(c_ang)
do
  call next_time_step(c_ang,p_ang,g,l,dt,n_ang)
  if(abs(c_ang-p_ang).ge.0.05D0) then
    call execute_command_line('cls')
    call display(c_ang)
  end if
end do
end program

subroutine next_time_step(c_ang,p_ang,g,l,dt,n_ang)
double precision :: c_ang, p_ang, g, l, dt, n_ang
n_ang = (-g*sin(c_ang)/l)*2.0D0*dt**2 + 2.0D0*c_ang - p_ang
p_ang = c_ang
c_ang = n_ang
end subroutine

subroutine display(c_ang)
double precision :: c_ang
character (len=*), parameter :: cfmt = '(A1)'
double precision :: rx, ry
integer :: x, y, i, j
rx = 45.0D0*sin(c_ang)
ry = 22.5D0*cos(c_ang)
x = int(rx)+51
y = int(ry)+2
do i = 1,32
  do j = 1,100
    if(i.eq.y .and. j.eq.x) then
      write(*,cfmt,advance='no') 'O'
    else if(i.eq.y .and. (j.eq.(x-1).or.j.eq.(x+1))) then
      write(*,cfmt,advance='no') 'G'
    else if(j.eq.x .and. (i.eq.(y-1).or.i.eq.(y+1))) then
      write(*,cfmt,advance='no') 'G'
    else if(i.eq.y .and. (j.eq.(x-2).or.j.eq.(x+2))) then
      write(*,cfmt,advance='no') '#'
    else if(j.eq.x .and. (i.eq.(y-2).or.i.eq.(y+2))) then
      write(*,cfmt,advance='no') 'G'
    else if((i.eq.(y+1).and.j.eq.(x+1)) .or. (i.eq.(y-1).and.j.eq.(x-1))) then
      write(*,cfmt,advance='no') '#'
    else if((i.eq.(y+1).and.j.eq.(x-1)) .or. (i.eq.(y-1).and.j.eq.(x+1))) then
      write(*,cfmt,advance='no') '#'
    else if(j.eq.50) then
      write(*,cfmt,advance='no') '|'
    else if(i.eq.2) then
      write(*,cfmt,advance='no') '-'
    else
      write(*,cfmt,advance='no') ' '
    end if
  end do
  write(*,*)
end do
end subroutine