subroutine sort(a, n)
c
c -- nonrecursive quick sort for vector a
c -- figure 6.7
c     
      integer n, i, j, sp, mid, left(20), right(20)
      real a(1), pivot
c
      left(1) = 1
      right(1) = n
      sp = 1
10    if (left(sp) .lt. right(sp)) goto 20
        sp = sp - 1
        goto 130
20     i = left(sp)
       j = right(sp)
       pivot = a(j)
       mid = (i + j) / 2
       if (j - i .lt. 6) goto 50
       if ((pivot .gt. a(i)).and.(pivot .lt. a(mid))) goto 50
       if ((pivot .lt. a(i)).and.(pivot .gt. a(mid))) goto 50
       if ((a(i) .lt. a(mid)).and.(a(i) .gt. pivot)) goto 30
       if ((a(i) .gt. a(mid)).and.(a(i) .lt. pivot)) goto 30
        call swap(a(mid), a(j))
        goto 40
30      call swap(a(i), a(j))
40     pivot = a(j)
50     if (i .ge. j) goto 110
60      if (a(i) .ge. pivot) goto 70
         i = i + 1
         goto 60
70      j = j - 1
80      if (.not.((i .lt. j) .and. (pivot .lt. a(j)))) goto 90
         j = j - 1
         goto 80
90      if (i .lt. j) call swap(a(i), a(j))
100    goto 50
110   j = right(sp)
      call swap(a(i), a(j))
      if ((i - left(sp)) .ge. (right(sp) - i)) goto 120
c -- stack shorter first
       left(sp+1) = left(sp)
       right(sp+1) = i - 1
       left(sp) = i + 1
       goto 125
120   left(sp+1) = i + 1
      right(sp+1) = right(sp)
      right(sp) = i - 1
125   sp = sp + 1
130   if (sp .gt. 0) goto 10
      return
      end