*DECK DROT SUBROUTINE DROT (N, DX, INCX, DY, INCY, DC, DS) C***BEGIN PROLOGUE DROT C***PURPOSE Apply a plane Givens rotation. C***LIBRARY SLATEC (BLAS) C***CATEGORY D1A8 C***TYPE DOUBLE PRECISION (SROT-S, DROT-D, CSROT-C) C***KEYWORDS BLAS, GIVENS ROTATION, GIVENS TRANSFORMATION, C LINEAR ALGEBRA, PLANE ROTATION, 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 DC D.P. element of rotation matrix C DS D.P. element of rotation matrix C C --Output-- C DX rotated vector DX (unchanged if N .LE. 0) C DY rotated vector DY (unchanged if N .LE. 0) C C Multiply the 2 x 2 matrix ( DC DS) times the 2 x N matrix (DX**T) C (-DS DC) (DY**T) C where **T indicates transpose. The elements of DX are in C DX(LX+I*INCX), I = 0 to N-1, where LX = 1 if INCX .GE. 0, else C LX = 1+(1-N)*INCX, and similarly for DY using LY and 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 861211 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 DROT DOUBLE PRECISION DX, DY, DC, DS, ZERO, ONE, W, Z DIMENSION DX(*), DY(*) SAVE ZERO, ONE DATA ZERO, ONE /0.0D0, 1.0D0/ C***FIRST EXECUTABLE STATEMENT DROT IF (N .LE. 0 .OR. (DS .EQ. ZERO .AND. DC .EQ. ONE)) GO TO 40 IF (.NOT. (INCX .EQ. INCY .AND. INCX .GT. 0)) GO TO 20 C C Code for equal and positive increments. C NSTEPS=INCX*N DO 10 I = 1,NSTEPS,INCX W=DX(I) Z=DY(I) DX(I)=DC*W+DS*Z DY(I)=-DS*W+DC*Z 10 CONTINUE GO TO 40 C C Code for unequal or nonpositive increments. C 20 CONTINUE KX=1 KY=1 C IF (INCX .LT. 0) KX = 1-(N-1)*INCX IF (INCY .LT. 0) KY = 1-(N-1)*INCY C DO 30 I = 1,N W=DX(KX) Z=DY(KY) DX(KX)=DC*W+DS*Z DY(KY)=-DS*W+DC*Z KX=KX+INCX KY=KY+INCY 30 CONTINUE 40 CONTINUE C RETURN END