ddot.f

Go to the documentation of this file.

*> \brief \b DDOT
*
*  =========== DOCUMENTATION ===========
*
* Online html documentation available at
*            http://www.netlib.org/lapack/explore-html/
*
*  Definition:
*  ===========
*
*       DOUBLE PRECISION FUNCTION DDOT(N,DX,INCX,DY,INCY)
*
*       .. Scalar Arguments ..
*       INTEGER INCX,INCY,N
*       ..
*       .. Array Arguments ..
*       DOUBLE PRECISION DX(*),DY(*)
*       ..
*
*
*> \par Purpose:
*  =============
*>
*> \verbatim
*>
*>    DDOT forms the dot product of two vectors.
*>    uses unrolled loops for increments equal to one.
*> \endverbatim
*
*  Arguments:
*  ==========
*
*> \param[in] N
*> \verbatim
*>          N is INTEGER
*>         number of elements in input vector(s)
*> \endverbatim
*>
*> \param[in] DX
*> \verbatim
*>          DX is DOUBLE PRECISION array, dimension ( 1 + ( N - 1 )*abs( INCX ) )
*> \endverbatim
*>
*> \param[in] INCX
*> \verbatim
*>          INCX is INTEGER
*>         storage spacing between elements of DX
*> \endverbatim
*>
*> \param[in] DY
*> \verbatim
*>          DY is DOUBLE PRECISION array, dimension ( 1 + ( N - 1 )*abs( INCY ) )
*> \endverbatim
*>
*> \param[in] INCY
*> \verbatim
*>          INCY is INTEGER
*>         storage spacing between elements of DY
*> \endverbatim
*
*  Authors:
*  ========
*
*> \author Univ. of Tennessee
*> \author Univ. of California Berkeley
*> \author Univ. of Colorado Denver
*> \author NAG Ltd.
*
*> \date November 2017
*
*> \ingroup double_blas_level1
*
*> \par Further Details:
*  =====================
*>
*> \verbatim
*>
*>     jack dongarra, linpack, 3/11/78.
*>     modified 12/3/93, array(1) declarations changed to array(*)
*> \endverbatim
*>
*  =====================================================================
      DOUBLE PRECISION FUNCTION ddot(N,DX,INCX,DY,INCY)
*
*  -- Reference BLAS level1 routine (version 3.8.0) --
*  -- Reference BLAS is a software package provided by Univ. of Tennessee,    --
*  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
*     November 2017
*
*     .. Scalar Arguments ..
      INTEGER INCX,INCY,N
*     ..
*     .. Array Arguments ..
      DOUBLE PRECISION DX(*),DY(*)
*     ..
*
*  =====================================================================
*
*     .. 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) THEN
*
*        code for both increments equal to 1
*
*
*        clean-up loop
*
         m = mod(n,5)
         IF (m.NE.0) THEN
            DO i = 1,m
               dtemp = dtemp + dx(i)*dy(i)
            END DO
            IF (n.LT.5) THEN
               ddot=dtemp
            RETURN
            END IF
         END IF
         mp1 = m + 1
         DO 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)
         END DO
      ELSE
*
*        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 i = 1,n
            dtemp = dtemp + dx(ix)*dy(iy)
            ix = ix + incx
            iy = iy + incy
         END DO
      END IF
      ddot = dtemp
      RETURN
      END