implicit none
      integer i,n
      real a,b,h,x,y,xl,yl,f,g
      real yp,k1,y0,z0
      real l1,zl,z,zp
      real err,exact
      character char*11


      print*,' Enter number of intervals n'
      read(5,*) n
      print*,' Enter a,b,y(a),dydx(a)'
      read(5,*) a,b,y0,z0

      h=(b-a)/float(n)


 100  continue
      y=y0
      z=z0
      x=a

      write(6,30) x,y,z
 30   format(/,' Solving with 4th Runge-Kutta gives:',/,
     &         '   X        Y         dY/dX     exact     %error',
     &         /,1x,f5.3,1x,f10.6,1x,f10.6)


      do i=1,n
        xl=x
        yl=y
        zl=z
        k1=h*f(xl,yl,zl)
        l1=h*g(xl,yl,zl)
        x=xl+h
        y=yl+k1
        z=zl+l1
        exact=sin(x)
        if(abs(exact).gt.1.e-6) then
           err=100.0*(y-exact)/exact
           write(6,2) x,y,z,exact,err
        else
           char='div by zero'
           write(6,3) x,y,z,exact,char
        endif

      enddo

 2    format(1x,f6.3,1x,f10.6,1x,f10.6,1x,f10.6,1x,f10.6)
 3    format(1x,f6.3,1x,f10.6,1x,f10.6,1x,f10.6,2x,a11)


      stop
      end


      real function f(x,y,z)
      implicit none
      real x,y,z
      f=z
      return
      end

      real function g(x,y,z)
      implicit none
      real x,y,z
      g=-y
      return
      end