d5/df6/ddot_8f_source.html

00001       DOUBLE PRECISION FUNCTION DDOT(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 *     DDOT forms the dot product of two vectors.
00013 *     uses unrolled loops for increments equal to 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       DDOT = 0.0d0
00031       DTEMP = 0.0d0
00032       IF (N.LE.0) RETURN
00033       IF (INCX.EQ.1 .AND. INCY.EQ.1) GO TO 20
00034 *
00035 *        code for unequal increments or equal increments
00036 *          not equal to 1
00037 *
00038       IX = 1
00039       IY = 1
00040       IF (INCX.LT.0) IX = (-N+1)*INCX + 1
00041       IF (INCY.LT.0) IY = (-N+1)*INCY + 1
00042       DO 10 I = 1,N
00043           DTEMP = DTEMP + DX(IX)*DY(IY)
00044           IX = IX + INCX
00045           IY = IY + INCY
00046    10 CONTINUE
00047       DDOT = DTEMP
00048       RETURN
00049 *
00050 *        code for both increments equal to 1
00051 *
00052 *
00053 *        clean-up loop
00054 *
00055    20 M = MOD(N,5)
00056       IF (M.EQ.0) GO TO 40
00057       DO 30 I = 1,M
00058           DTEMP = DTEMP + DX(I)*DY(I)
00059    30 CONTINUE
00060       IF (N.LT.5) GO TO 60
00061    40 MP1 = M + 1
00062       DO 50 I = MP1,N,5
00063           DTEMP = DTEMP + DX(I)*DY(I) + DX(I+1)*DY(I+1) +
00064      +            DX(I+2)*DY(I+2) + DX(I+3)*DY(I+3) + DX(I+4)*DY(I+4)
00065    50 CONTINUE
00066    60 DDOT = DTEMP
00067       RETURN
00068       END