db/dd4/dswap_8f_source.html

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) GO TO 20
00032 *
00033 *       code for unequal increments or equal increments not equal
00034 *         to 1
00035 *
00036       IX = 1
00037       IY = 1
00038       IF (INCX.LT.0) IX = (-N+1)*INCX + 1
00039       IF (INCY.LT.0) IY = (-N+1)*INCY + 1
00040       DO 10 I = 1,N
00041           DTEMP = DX(IX)
00042           DX(IX) = DY(IY)
00043           DY(IY) = DTEMP
00044           IX = IX + INCX
00045           IY = IY + INCY
00046    10 CONTINUE
00047       RETURN
00048 *
00049 *       code for both increments equal to 1
00050 *
00051 *
00052 *       clean-up loop
00053 *
00054    20 M = MOD(N,3)
00055       IF (M.EQ.0) GO TO 40
00056       DO 30 I = 1,M
00057           DTEMP = DX(I)
00058           DX(I) = DY(I)
00059           DY(I) = DTEMP
00060    30 CONTINUE
00061       IF (N.LT.3) RETURN
00062    40 MP1 = M + 1
00063       DO 50 I = MP1,N,3
00064           DTEMP = DX(I)
00065           DX(I) = DY(I)
00066           DY(I) = DTEMP
00067           DTEMP = DX(I+1)
00068           DX(I+1) = DY(I+1)
00069           DY(I+1) = DTEMP
00070           DTEMP = DX(I+2)
00071           DX(I+2) = DY(I+2)
00072           DY(I+2) = DTEMP
00073    50 CONTINUE
00074       RETURN
00075       END