Skip to content

HTTPS clone URL

Subversion checkout URL

You can clone with
or
.
Download ZIP
tree: 36c9f36980
Fetching contributors…

Cannot retrieve contributors at this time

90 lines (89 sloc) 2.883 kB
!DECK DDOT
DOUBLE PRECISION FUNCTION DDOT (N, DX, INCX, DY, INCY)
!***BEGIN PROLOGUE DDOT
!***PURPOSE Compute the inner product of two vectors.
!***LIBRARY SLATEC (BLAS)
!***CATEGORY D1A4
!***TYPE DOUBLE PRECISION (SDOT-S, DDOT-D, CDOTU-C)
!***KEYWORDS BLAS, INNER PRODUCT, LINEAR ALGEBRA, VECTOR
!***AUTHOR Lawson, C. L., (JPL)
! Hanson, R. J., (SNLA)
! Kincaid, D. R., (U. of Texas)
! Krogh, F. T., (JPL)
!***DESCRIPTION
!
! B L A S Subprogram
! Description of Parameters
!
! --Input--
! N number of elements in input vector(s)
! DX double precision vector with N elements
! INCX storage spacing between elements of DX
! DY double precision vector with N elements
! INCY storage spacing between elements of DY
!
! --Output--
! DDOT double precision dot product (zero if N .LE. 0)
!
! Returns the dot product of double precision DX and DY.
! DDOT = sum for I = 0 to N-1 of DX(LX+I*INCX) * DY(LY+I*INCY),
! where LX = 1 if INCX .GE. 0, else LX = 1+(1-N)*INCX, and LY is
! defined in a similar way using INCY.
!
!***REFERENCES C. L. Lawson, R. J. Hanson, D. R. Kincaid and F. T.
! Krogh, Basic linear algebra subprograms for Fortran
! usage, Algorithm No. 539, Transactions on Mathematical
! Software 5, 3 (September 1979), pp. 308-323.
!***ROUTINES CALLED (NONE)
!***REVISION HISTORY (YYMMDD)
! 791001 DATE WRITTEN
! 890831 Modified array declarations. (WRB)
! 890831 REVISION DATE from Version 3.2
! 891214 Prologue converted to Version 4.0 format. (BAB)
! 920310 Corrected definition of LX in DESCRIPTION. (WRB)
! 920501 Reformatted the REFERENCES section. (WRB)
!***END PROLOGUE DDOT
DOUBLE PRECISION DX(*), DY(*)
!***FIRST EXECUTABLE STATEMENT DDOT
DDOT = 0.0D0
IF (N .LE. 0) RETURN
IF (INCX .EQ. INCY) IF (INCX-1) 5,20,60
!
! Code for unequal or nonpositive increments.
!
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
!
! Code for both increments equal to 1.
!
! Clean-up loop so remaining vector length is a multiple of 5.
!
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) + &
DX(I+3)*DY(I+3) + DX(I+4)*DY(I+4)
50 CONTINUE
RETURN
!
! Code for equal, positive, non-unit increments.
!
60 NS = N*INCX
DO 70 I = 1,NS,INCX
DDOT = DDOT + DX(I)*DY(I)
70 CONTINUE
RETURN
END
Jump to Line
Something went wrong with that request. Please try again.