relibc_openlibm/slatec/dsdot.f

*DECK DSDOT
      DOUBLE PRECISION FUNCTION DSDOT (N, SX, INCX, SY, INCY)
C***BEGIN PROLOGUE  DSDOT
C***PURPOSE  Compute the inner product of two vectors with extended
C            precision accumulation and result.
C***LIBRARY   SLATEC (BLAS)
C***CATEGORY  D1A4
C***TYPE      DOUBLE PRECISION (DSDOT-D, DCDOT-C)
C***KEYWORDS  BLAS, COMPLEX VECTORS, DOT PRODUCT, INNER PRODUCT,
C             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       SX  single precision vector with N elements
C     INCX  storage spacing between elements of SX
C       SY  single precision vector with N elements
C     INCY  storage spacing between elements of SY
C
C     --Output--
C    DSDOT  double precision dot product (zero if N.LE.0)
C
C     Returns D.P. dot product accumulated in D.P., for S.P. SX and SY
C     DSDOT = sum for I = 0 to N-1 of  SX(LX+I*INCX) * SY(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  DSDOT
      REAL SX(*),SY(*)
C***FIRST EXECUTABLE STATEMENT  DSDOT
      DSDOT = 0.0D0
      IF (N .LE. 0) RETURN
      IF (INCX.EQ.INCY .AND. INCX.GT.0) GO TO 20
C
C     Code for unequal or nonpositive increments.
C
      KX = 1
      KY = 1
      IF (INCX .LT. 0) KX = 1+(1-N)*INCX
      IF (INCY .LT. 0) KY = 1+(1-N)*INCY
      DO 10 I = 1,N
        DSDOT = DSDOT + DBLE(SX(KX))*DBLE(SY(KY))
        KX = KX + INCX
        KY = KY + INCY
   10 CONTINUE
      RETURN
C
C     Code for equal, positive, non-unit increments.
C
   20 NS = N*INCX
      DO 30 I = 1,NS,INCX
        DSDOT = DSDOT + DBLE(SX(I))*DBLE(SY(I))
   30 CONTINUE
      RETURN
      END