*DECK DDOT
DOUBLE PRECISION FUNCTION DDOT (N, DX, INCX, DY, INCY)
C***BEGIN PROLOGUE DDOT
C***PURPOSE Compute the inner product of two vectors.
C***LIBRARY SLATEC (BLAS)
C***CATEGORY D1A4
C***TYPE DOUBLE PRECISION (SDOT-S, DDOT-D, CDOTU-C)
C***KEYWORDS BLAS, INNER PRODUCT, LINEAR ALGEBRA, VECTOR
C***AUTHOR Lawson, C. L., (JPL)
C Hanson, R. J., (SNLA)
C Kincaid, D. R., (U. of Texas)
C Krogh, F. T., (JPL)
C***DESCRIPTION
C
C B L A S Subprogram
C Description of Parameters
C
C --Input--
C N number of elements in input vector(s)
C DX double precision vector with N elements
C INCX storage spacing between elements of DX
C DY double precision vector with N elements
C INCY storage spacing between elements of DY
C
C --Output--
C DDOT double precision dot product (zero if N .LE. 0)
C
C Returns the dot product of double precision DX and DY.
C DDOT = sum for I = 0 to N-1 of DX(LX+I*INCX) * DY(LY+I*INCY),
C where LX = 1 if INCX .GE. 0, else LX = 1+(1-N)*INCX, and LY is
C defined in a similar way using INCY.
C
C***REFERENCES C. L. Lawson, R. J. Hanson, D. R. Kincaid and F. T.
C Krogh, Basic linear algebra subprograms for Fortran
C usage, Algorithm No. 539, Transactions on Mathematical
C Software 5, 3 (September 1979), pp. 308-323.
C***ROUTINES CALLED (NONE)
C***REVISION HISTORY (YYMMDD)
C 791001 DATE WRITTEN
C 890831 Modified array declarations. (WRB)
C 890831 REVISION DATE from Version 3.2
C 891214 Prologue converted to Version 4.0 format. (BAB)
C 920310 Corrected definition of LX in DESCRIPTION. (WRB)
C 920501 Reformatted the REFERENCES section. (WRB)
C***END PROLOGUE DDOT
DOUBLE PRECISION DX(*), DY(*)
C***FIRST EXECUTABLE STATEMENT DDOT
DDOT = 0.0D0
IF (N .LE. 0) RETURN
IF (INCX .EQ. INCY) IF (INCX-1) 5,20,60
C
C Code for unequal or nonpositive increments.
C
5 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
DDOT = DDOT + DX(IX)*DY(IY)
IX = IX + INCX
IY = IY + INCY
10 CONTINUE
RETURN
C
C Code for both increments equal to 1.
C
C Clean-up loop so remaining vector length is a multiple of 5.
C
20 M = MOD(N,5)
IF (M .EQ. 0) GO TO 40
DO 30 I = 1,M
DDOT = DDOT + DX(I)*DY(I)
30 CONTINUE
IF (N .LT. 5) RETURN
40 MP1 = M + 1
DO 50 I = MP1,N,5
DDOT = DDOT + DX(I)*DY(I) + DX(I+1)*DY(I+1) + DX(I+2)*DY(I+2) +
1 DX(I+3)*DY(I+3) + DX(I+4)*DY(I+4)
50 CONTINUE
RETURN
C
C Code for equal, positive, non-unit increments.
C
60 NS = N*INCX
DO 70 I = 1,NS,INCX
DDOT = DDOT + DX(I)*DY(I)
70 CONTINUE
RETURN
END