2 DOUBLE PRECISION FUNCTION DDOT (N, DX, INCX, DY, INCY)
3 C***BEGIN PROLOGUE DDOT
4 C***PURPOSE Compute the inner product of two vectors.
5 C***LIBRARY SLATEC (BLAS)
7 C***TYPE DOUBLE PRECISION (SDOT-S, DDOT-D, CDOTU-C)
8 C***KEYWORDS BLAS, INNER PRODUCT, LINEAR ALGEBRA, VECTOR
9 C***AUTHOR Lawson, C. L., (JPL)
10 C Hanson, R. J., (SNLA)
11 C Kincaid, D. R., (U. of Texas)
16 C Description of Parameters
19 C N number of elements in input vector(s)
20 C DX double precision vector with N elements
21 C INCX storage spacing between elements of DX
22 C DY double precision vector with N elements
23 C INCY storage spacing between elements of DY
26 C DDOT double precision dot product (zero if N .LE. 0)
28 C Returns the dot product of double precision DX and DY.
29 C DDOT = sum for I = 0 to N-1 of DX(LX+I*INCX) * DY(LY+I*INCY),
30 C where LX = 1 if INCX .GE. 0, else LX = 1+(1-N)*INCX, and LY is
31 C defined in a similar way using INCY.
33 C***REFERENCES C. L. Lawson, R. J. Hanson, D. R. Kincaid and F. T.
34 C Krogh, Basic linear algebra subprograms for Fortran
35 C usage, Algorithm No. 539, Transactions on Mathematical
36 C Software 5, 3 (September 1979), pp. 308-323.
37 C***ROUTINES CALLED (NONE)
38 C***REVISION HISTORY (YYMMDD)
40 C 890831 Modified array declarations. (WRB)
41 C 890831 REVISION DATE from Version 3.2
42 C 891214 Prologue converted to Version 4.0 format. (BAB)
43 C 920310 Corrected definition of LX in DESCRIPTION. (WRB)
44 C 920501 Reformatted the REFERENCES section. (WRB)
46 DOUBLE PRECISION DX(*), DY(*)
47 C***FIRST EXECUTABLE STATEMENT DDOT
51 IF (INCX .EQ. INCY) IF (INCX-1) 5,20,60
53 C Code for unequal or nonpositive increments.
57 IF (INCX .LT. 0) IX = (-N+1)*INCX + 1
58 IF (INCY .LT. 0) IY = (-N+1)*INCY + 1
60 DDOT = DDOT + DX(IX)*DY(IY)
66 C Code for both increments equal to 1.
68 C Clean-up loop so remaining vector length is a multiple of 5.
71 IF (M .EQ. 0) GO TO 40
73 DDOT = DDOT + DX(I)*DY(I)
78 DDOT = DDOT + DX(I)*DY(I) + DX(I+1)*DY(I+1) + DX(I+2)*DY(I+2) +
79 1 DX(I+3)*DY(I+3) + DX(I+4)*DY(I+4)
83 C Code for equal, positive, non-unit increments.
87 DDOT = DDOT + DX(I)*DY(I)