C     @(#)exchange.f	1.2     03/25/98
C
      subroutine exchange (a, old, new)

c Exchanges occurrences of 'OLD' for 'NEW' within 'A'

c NOTE: This will not work on PC or SUN if LEN(NEW) > LEN(OLD)

      character*(*) a, old, new

      if(len(new).gt.len(old)) call abort('exchange: new longer than old')


      in = index (a, old)
      if (in .eq. 1) then
        if (in+len(old) .eq. len(a)+1) then
          a = new
        else
          a = new//a(in+len(old):len(a))
        end if
        in = index (a, old)
      end if

c now 'in' can no longer have the value 1

      do while (in .ne. 0)
        if (in+len(old) .eq. len(a)+1) then
          a = a(1:in-1)//new
        else
          a = a(1:in-1)//new//a(in+len(old):len(a))
        end if
        in = index (a, old)
      end do
      return
      end