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En route vers la 4.4.1.

    1:       SUBROUTINE DSWAP(N,DX,INCX,DY,INCY)
    2: *     .. Scalar Arguments ..
    3:       INTEGER INCX,INCY,N
    4: *     ..
    5: *     .. Array Arguments ..
    6:       DOUBLE PRECISION DX(*),DY(*)
    7: *     ..
    8: *
    9: *  Purpose
   10: *  =======
   11: *
   12: *     interchanges two vectors.
   13: *     uses unrolled loops for increments equal one.
   14: *
   15: *  Further Details
   16: *  ===============
   17: *
   18: *     jack dongarra, linpack, 3/11/78.
   19: *     modified 12/3/93, array(1) declarations changed to array(*)
   20: *
   21: *  =====================================================================
   22: *
   23: *     .. Local Scalars ..
   24:       DOUBLE PRECISION DTEMP
   25:       INTEGER I,IX,IY,M,MP1
   26: *     ..
   27: *     .. Intrinsic Functions ..
   28:       INTRINSIC MOD
   29: *     ..
   30:       IF (N.LE.0) RETURN
   31:       IF (INCX.EQ.1 .AND. INCY.EQ.1) THEN
   32: *
   33: *       code for both increments equal to 1
   34: *
   35: *
   36: *       clean-up loop
   37: *
   38:          M = MOD(N,3)
   39:          IF (M.NE.0) THEN
   40:             DO I = 1,M
   41:                DTEMP = DX(I)
   42:                DX(I) = DY(I)
   43:                DY(I) = DTEMP
   44:             END DO
   45:             IF (N.LT.3) RETURN
   46:          END IF
   47:          MP1 = M + 1
   48:          DO I = MP1,N,3
   49:             DTEMP = DX(I)
   50:             DX(I) = DY(I)
   51:             DY(I) = DTEMP
   52:             DTEMP = DX(I+1)
   53:             DX(I+1) = DY(I+1)
   54:             DY(I+1) = DTEMP
   55:             DTEMP = DX(I+2)
   56:             DX(I+2) = DY(I+2)
   57:             DY(I+2) = DTEMP
   58:          END DO
   59:       ELSE
   60: *
   61: *       code for unequal increments or equal increments not equal
   62: *         to 1
   63: *
   64:          IX = 1
   65:          IY = 1
   66:          IF (INCX.LT.0) IX = (-N+1)*INCX + 1
   67:          IF (INCY.LT.0) IY = (-N+1)*INCY + 1
   68:          DO I = 1,N
   69:             DTEMP = DX(IX)
   70:             DX(IX) = DY(IY)
   71:             DY(IY) = DTEMP
   72:             IX = IX + INCX
   73:             IY = IY + INCY
   74:          END DO
   75:       END IF
   76:       RETURN
   77:       END