c     Fortran 77 program to return
c       all _r_ combinations of _n_
c
c       converted from F95 array subscripting
c
c     Written by Jeffrey Ryan
c     jeff _dot_ a _dot_ ryan _at_ gmail _dot_ com
c     Copyright 2007
c     Distributed under the MIT-style license

      subroutine ncr(n,r,b,a,ret)
c     Usage:
c 
c     n   integer number of elements to choose from 
c     r   integer number of elements chosen
c     b   set of integer elements to choose from 
c     a   all.comb flag: 1 - show all, 0 - show next
c     ret return array of integers
c
      integer n,r,b(r),a,ret(*)
      integer cout(r),cl,mx,w,pos
c 
      pos=1
      cout = b
c     F95 version: ret(pos:r)  = b
c     F77 do loop instead
      do 10 i=1,r
        ret(i) = b(i)
   10 continue
c     cl   current column
c     mx   column maximum value
      cl=r
      mx=n
    1 do 20 while(1 .eq. 1)
c
c       if can increase column, do so
        if(cout(cl) .lt. mx) then
          w = cout(cl)
          do i=0,(r-cl)
            cout(cl+i) = w+i+1
          end do
          cl = r
          mx = n
          pos = pos + r
c         F95 version: ret(pos:(pos+r-1)) = cout
c         F77 do loop instead
          j = 1
          do 30 i=pos,(pos+r-1)
            ret(i) = cout(j)
            j = j + 1
   30     continue
          if(a .eq. 0) exit
c         F95 version: cycle
c         F77 use goto statement instead
          goto 1
        end if
c       if can't increase column, move left
        cl = cl - 1
c       and decrease maximum by one
        mx = mx - 1
c       if 
        if(cl .eq. 0) exit
   20 continue
      end