program heat2

      double precision u(101),dx,dt,t
      integer i,j,count
      dx = 0.2
      dt = 0.01
      count = 0
      open(unit=13,file="data.d")

      u(1) = 2
      u(101) = 3
      do 10 i=2,100
         if(i .LT. 50) then
            u(i) = 1
         else
            u(i) = 0
         endif
 10   continue

      write(13,900) (u(j), j=1,101)

      do 20 t=0,50,dt
         write(*,*) t
         do 30 i=2,100
            u(i) = u(i) + ((u(i-1)-2*u(i)+u(i+1))/dx**2)*dt
 30      continue
         if(count .GE. 100) then
            write(13,900) (u(j), j=1,101)
            count = 0
         else
            count = count + 1
         endif
 20   continue

 900  format(101(F5.2))
      stop 
      end