subroutine  dswapf (n,dx,incx,dy,incy)

c

c     interchanges two vectors.

c     uses unrolled loops for increments equal one.

c     jack dongarra, linpack, 3/11/78.

c     modified 12/3/93, array(1) declarations changed to array(*)

c

      double precision dx(*),dy(*),dtemp

      integer i,incx,incy,ix,iy,m,mp1,n

c

      if(n.le.0)return

      if(incx.eq.1.and.incy.eq.1)go to 20

c

c       code for unequal increments or equal increments not equal

c         to 1

c

      ix = 1

      iy = 1

      if(incx.lt.0)ix = (-n+1)*incx + 1

      if(incy.lt.0)iy = (-n+1)*incy + 1

      do 10 i = 1,n

        dtemp = dx(ix)

        dx(ix) = dy(iy)

        dy(iy) = dtemp

        ix = ix + incx

        iy = iy + incy

   10 continue

      return

c

c       code for both increments equal to 1

c

c

c       clean-up loop

c

   20 m = mod(n,3)

      if( m .eq. 0 ) go to 40

      do 30 i = 1,m

        dtemp = dx(i)

        dx(i) = dy(i)

        dy(i) = dtemp

   30 continue

      if( n .lt. 3 ) return

   40 mp1 = m + 1

      do 50 i = mp1,n,3

        dtemp = dx(i)

        dx(i) = dy(i)

        dy(i) = dtemp

        dtemp = dx(i + 1)

        dx(i + 1) = dy(i + 1)

        dy(i + 1) = dtemp

        dtemp = dx(i + 2)

        dx(i + 2) = dy(i + 2)

        dy(i + 2) = dtemp

   50 continue

      return

      end