LAPACK 3.3.1 Linear Algebra PACKage

# dswap.f

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```00001       SUBROUTINE DSWAP(N,DX,INCX,DY,INCY)
00002 *     .. Scalar Arguments ..
00003       INTEGER INCX,INCY,N
00004 *     ..
00005 *     .. Array Arguments ..
00006       DOUBLE PRECISION DX(*),DY(*)
00007 *     ..
00008 *
00009 *  Purpose
00010 *  =======
00011 *
00012 *     interchanges two vectors.
00013 *     uses unrolled loops for increments equal one.
00014 *
00015 *  Further Details
00016 *  ===============
00017 *
00018 *     jack dongarra, linpack, 3/11/78.
00019 *     modified 12/3/93, array(1) declarations changed to array(*)
00020 *
00021 *  =====================================================================
00022 *
00023 *     .. Local Scalars ..
00024       DOUBLE PRECISION DTEMP
00025       INTEGER I,IX,IY,M,MP1
00026 *     ..
00027 *     .. Intrinsic Functions ..
00028       INTRINSIC MOD
00029 *     ..
00030       IF (N.LE.0) RETURN
00031       IF (INCX.EQ.1 .AND. INCY.EQ.1) THEN
00032 *
00033 *       code for both increments equal to 1
00034 *
00035 *
00036 *       clean-up loop
00037 *
00038          M = MOD(N,3)
00039          IF (M.NE.0) THEN
00040             DO I = 1,M
00041                DTEMP = DX(I)
00042                DX(I) = DY(I)
00043                DY(I) = DTEMP
00044             END DO
00045             IF (N.LT.3) RETURN
00046          END IF
00047          MP1 = M + 1
00048          DO I = MP1,N,3
00049             DTEMP = DX(I)
00050             DX(I) = DY(I)
00051             DY(I) = DTEMP
00052             DTEMP = DX(I+1)
00053             DX(I+1) = DY(I+1)
00054             DY(I+1) = DTEMP
00055             DTEMP = DX(I+2)
00056             DX(I+2) = DY(I+2)
00057             DY(I+2) = DTEMP
00058          END DO
00059       ELSE
00060 *
00061 *       code for unequal increments or equal increments not equal
00062 *         to 1
00063 *
00064          IX = 1
00065          IY = 1
00066          IF (INCX.LT.0) IX = (-N+1)*INCX + 1
00067          IF (INCY.LT.0) IY = (-N+1)*INCY + 1
00068          DO I = 1,N
00069             DTEMP = DX(IX)
00070             DX(IX) = DY(IY)
00071             DY(IY) = DTEMP
00072             IX = IX + INCX
00073             IY = IY + INCY
00074          END DO
00075       END IF
00076       RETURN
00077       END
```