LAPACK  3.8.0
LAPACK: Linear Algebra PACKage

◆ ddot()

double precision function ddot ( integer  N,
double precision, dimension(*)  DX,
integer  INCX,
double precision, dimension(*)  DY,
integer  INCY 
)

DDOT

Purpose:
    DDOT forms the dot product of two vectors.
    uses unrolled loops for increments equal to one.
Parameters
[in]N
          N is INTEGER
         number of elements in input vector(s)
[in]DX
          DX is DOUBLE PRECISION array, dimension ( 1 + ( N - 1 )*abs( INCX ) )
[in]INCX
          INCX is INTEGER
         storage spacing between elements of DX
[in]DY
          DY is DOUBLE PRECISION array, dimension ( 1 + ( N - 1 )*abs( INCY ) )
[in]INCY
          INCY is INTEGER
         storage spacing between elements of DY
Author
Univ. of Tennessee
Univ. of California Berkeley
Univ. of Colorado Denver
NAG Ltd.
Date
November 2017
Further Details:
     jack dongarra, linpack, 3/11/78.
     modified 12/3/93, array(1) declarations changed to array(*)

Definition at line 84 of file ddot.f.

84 *
85 * -- Reference BLAS level1 routine (version 3.8.0) --
86 * -- Reference BLAS is a software package provided by Univ. of Tennessee, --
87 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
88 * November 2017
89 *
90 * .. Scalar Arguments ..
91  INTEGER incx,incy,n
92 * ..
93 * .. Array Arguments ..
94  DOUBLE PRECISION dx(*),dy(*)
95 * ..
96 *
97 * =====================================================================
98 *
99 * .. Local Scalars ..
100  DOUBLE PRECISION dtemp
101  INTEGER i,ix,iy,m,mp1
102 * ..
103 * .. Intrinsic Functions ..
104  INTRINSIC mod
105 * ..
106  ddot = 0.0d0
107  dtemp = 0.0d0
108  IF (n.LE.0) RETURN
109  IF (incx.EQ.1 .AND. incy.EQ.1) THEN
110 *
111 * code for both increments equal to 1
112 *
113 *
114 * clean-up loop
115 *
116  m = mod(n,5)
117  IF (m.NE.0) THEN
118  DO i = 1,m
119  dtemp = dtemp + dx(i)*dy(i)
120  END DO
121  IF (n.LT.5) THEN
122  ddot=dtemp
123  RETURN
124  END IF
125  END IF
126  mp1 = m + 1
127  DO i = mp1,n,5
128  dtemp = dtemp + dx(i)*dy(i) + dx(i+1)*dy(i+1) +
129  $ dx(i+2)*dy(i+2) + dx(i+3)*dy(i+3) + dx(i+4)*dy(i+4)
130  END DO
131  ELSE
132 *
133 * code for unequal increments or equal increments
134 * not equal to 1
135 *
136  ix = 1
137  iy = 1
138  IF (incx.LT.0) ix = (-n+1)*incx + 1
139  IF (incy.LT.0) iy = (-n+1)*incy + 1
140  DO i = 1,n
141  dtemp = dtemp + dx(ix)*dy(iy)
142  ix = ix + incx
143  iy = iy + incy
144  END DO
145  END IF
146  ddot = dtemp
147  RETURN
double precision function ddot(N, DX, INCX, DY, INCY)
DDOT
Definition: ddot.f:84
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