program cissoid_of_diocles
      implicit none
      integer pgopen, i
      real x1(3600), x2(3600), r, theta, delta_theta, c, pi

      if (pgopen('/xwin').le.0) stop

      write(*,*) 'Please input a non-zero real number "c" (ex. 1.0) :'
      read(*,*) c

      pi = 3.14159268
      delta_theta = 2*pi/3600
      
      do i=1, 3600
        theta = (i-1)*delta_theta
        r = 2*c*sin(theta)*sin(theta)/cos(theta)
        x1(i)=r*cos(theta)
        x2(i)=r*sin(theta)
      end do

      call pgenv(-2.0,2.0,-2.0,2.0,1,0)
      call pglab('x1','x2','Cissoid of Diocles')
      call pgline(3600,x1,x2)
      call pgclos

      end