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  1. C DSWAP SOURCE BP208322 15/10/13 21:15:58 8670
  2. subroutine dswap (n,dx,incx,dy,incy)
  3. c
  4. c interchanges two vectors.
  5. c uses unrolled loops for increments equal one.
  6. c jack dongarra, linpack, 3/11/78.
  7. c
  8. REAL*8 dx(1),dy(1),dtemp
  9. integer i,incx,incy,ix,iy,m,mp1,n
  10. c
  11. if(n.le.0)return
  12. if(incx.eq.1.and.incy.eq.1)go to 20
  13. c
  14. c code for unequal increments or equal increments not equal
  15. c to 1
  16. c
  17. ix = 1
  18. iy = 1
  19. if(incx.lt.0)ix = (-n+1)*incx + 1
  20. if(incy.lt.0)iy = (-n+1)*incy + 1
  21. do 10 i = 1,n
  22. dtemp = dx(ix)
  23. dx(ix) = dy(iy)
  24. dy(iy) = dtemp
  25. ix = ix + incx
  26. iy = iy + incy
  27. 10 continue
  28. return
  29. c
  30. c code for both increments equal to 1
  31. c
  32. c
  33. c clean-up loop
  34. c
  35. 20 m = mod(n,3)
  36. if( m .eq. 0 ) go to 40
  37. do 30 i = 1,m
  38. dtemp = dx(i)
  39. dx(i) = dy(i)
  40. dy(i) = dtemp
  41. 30 continue
  42. if( n .lt. 3 ) return
  43. 40 mp1 = m + 1
  44. do 50 i = mp1,n,3
  45. dtemp = dx(i)
  46. dx(i) = dy(i)
  47. dy(i) = dtemp
  48. dtemp = dx(i + 1)
  49. dx(i + 1) = dy(i + 1)
  50. dy(i + 1) = dtemp
  51. dtemp = dx(i + 2)
  52. dx(i + 2) = dy(i + 2)
  53. dy(i + 2) = dtemp
  54. 50 continue
  55. return
  56. end
  57.  
  58.  

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