*DECK DAXPY SUBROUTINE DAXPY (N, DA, DX, INCX, DY, INCY) C***BEGIN PROLOGUE DAXPY C***PURPOSE Compute a constant times a vector plus a vector. C***LIBRARY SLATEC (BLAS) C***CATEGORY D1A7 C***TYPE DOUBLE PRECISION (SAXPY-S, DAXPY-D, CAXPY-C) C***KEYWORDS BLAS, LINEAR ALGEBRA, TRIAD, 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 DA double precision scalar multiplier 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 DY double precision result (unchanged if N .LE. 0) C C Overwrite double precision DY with double precision DA*DX + DY. C For I = 0 to N-1, replace DY(LY+I*INCY) with DA*DX(LX+I*INCX) + C 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 DAXPY DOUBLE PRECISION DX(*), DY(*), DA C***FIRST EXECUTABLE STATEMENT DAXPY IF (N.LE.0 .OR. DA.EQ.0.0D0) 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 DY(IY) = DY(IY) + DA*DX(IX) 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 4. C 20 M = MOD(N,4) IF (M .EQ. 0) GO TO 40 DO 30 I = 1,M DY(I) = DY(I) + DA*DX(I) 30 CONTINUE IF (N .LT. 4) RETURN 40 MP1 = M + 1 DO 50 I = MP1,N,4 DY(I) = DY(I) + DA*DX(I) DY(I+1) = DY(I+1) + DA*DX(I+1) DY(I+2) = DY(I+2) + DA*DX(I+2) DY(I+3) = DY(I+3) + DA*DX(I+3) 50 CONTINUE RETURN C C Code for equal, positive, non-unit increments. C 60 NS = N*INCX DO 70 I = 1,NS,INCX DY(I) = DA*DX(I) + DY(I) 70 CONTINUE RETURN END