ddot.f

贝尔实验室多年开发的矩阵计算程序库的说明文件

      DOUBLE PRECISION FUNCTION DDOT(N,DX,INCX,DY,INCY)
*     .. Scalar Arguments ..
      INTEGER INCX,INCY,N
*     ..
*     .. Array Arguments ..
      DOUBLE PRECISION DX(*),DY(*)
*     ..
*
*  Purpose
*  =======
*
*     forms the dot product of two vectors.
*     uses unrolled loops for increments equal to one.
*     jack dongarra, linpack, 3/11/78.
*     modified 12/3/93, array(1) declarations changed to array(*)
*
*
*     .. Local Scalars ..
      DOUBLE PRECISION DTEMP
      INTEGER I,IX,IY,M,MP1
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC MOD
*     ..
      DDOT = 0.0d0
      DTEMP = 0.0d0
      IF (N.LE.0) RETURN
      IF (INCX.EQ.1 .AND. INCY.EQ.1) GO TO 20
*
*        code for unequal increments or equal increments
*          not equal to 1
*
      IX = 1
      IY = 1
      IF (INCX.LT.0) IX = (-N+1)*INCX + 1
      IF (INCY.LT.0) IY = (-N+1)*INCY + 1
      DO 10 I = 1,N
          DTEMP = DTEMP + DX(IX)*DY(IY)
          IX = IX + INCX
          IY = IY + INCY
   10 CONTINUE
      DDOT = DTEMP
      RETURN
*
*        code for both increments equal to 1
*
*
*        clean-up loop
*
   20 M = MOD(N,5)
      IF (M.EQ.0) GO TO 40
      DO 30 I = 1,M
          DTEMP = DTEMP + DX(I)*DY(I)
   30 CONTINUE
      IF (N.LT.5) GO TO 60
   40 MP1 = M + 1
      DO 50 I = MP1,N,5
          DTEMP = DTEMP + DX(I)*DY(I) + DX(I+1)*DY(I+1) +
     +            DX(I+2)*DY(I+2) + DX(I+3)*DY(I+3) + DX(I+4)*DY(I+4)
   50 CONTINUE
   60 DDOT = DTEMP
      RETURN
      END