      subroutine iswap (n,sx,incx,sy,incy)
c
c     interchanges two vectors.
c     uses unrolled loops for increments equal to 1.
c     jack dongarra, linpack, 3/11/78.
c
      integer sx(1),sy(1),stemp
      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
        stemp = sx(ix)
        sx(ix) = sy(iy)
        sy(iy) = stemp
        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
        stemp = sx(i)
        sx(i) = sy(i)
        sy(i) = stemp
   30 continue
      if( n .lt. 3 ) return
   40 mp1 = m + 1
      do 50 i = mp1,n,3
        stemp = sx(i)
        sx(i) = sy(i)
        sy(i) = stemp
        stemp = sx(i + 1)
        sx(i + 1) = sy(i + 1)
        sy(i + 1) = stemp
        stemp = sx(i + 2)
        sx(i + 2) = sy(i + 2)
        sy(i + 2) = stemp
   50 continue
      return
      end
