program program7
C
C  this program demonstrates DO loops
C
C  1/30/2005 CWJ SDSU
C

      implicit none
 
      integer i,j       ! dummy indices
      integer istart,iend    ! start, stop of do loop
      integer iskip   ! skipping in do loop
      integer sum     ! sum of integers

      print*, ' Demo of DO loops '
      print*, ' Enter start, stop '
      read*,istart,iend

C  initialize SUM
      sum = 0
C  do first DO loop
      do i = istart,iend
        print*,i
        sum =sum+i
      enddo
      print*,' sum = ',sum

C     now illustrate skipping
C  (use alternate input/output commands)
 
      write(6,*)' Enter # steps to skip '
      read(5,*)iskip

C----- re-initialize SUM

      sum =0
      do i = istart,iend,iskip
        print*,i
        sum = sum+i
      enddo
      print*,' sum now = ',sum

C--------------- now for nested loops ----------

      print*,' '
      print*,' nested do loop: '
      do i = 1,4
        do j = 1,4
          print*,' i = ',i,', j = ',j
        enddo     ! do-loop over j
      enddo       ! do-loop over i


      end