share/lapack/blas/fortran/drotg.f

   1       subroutine drotg(da,db,c,s)
   2 c
   3 c     construct givens plane rotation.
   4 c     jack dongarra, linpack, 3/11/78.
   5 c
   6       double precision da,db,c,s,roe,scale,r,z
   7 c
   8       roe = db
   9       if( dabs(da) .gt. dabs(db) ) roe = da
  10       scale = dabs(da) + dabs(db)
  11       if( scale .ne. 0.0d0 ) go to 10
  12          c = 1.0d0
  13          s = 0.0d0
  14          r = 0.0d0
  15          z = 0.0d0
  16          go to 20
  17    10 r = scale*dsqrt((da/scale)**2 + (db/scale)**2)
  18       r = dsign(1.0d0,roe)*r
  19       c = da/r
  20       s = db/r
  21       z = 1.0d0
  22       if( dabs(da) .gt. dabs(db) ) z = s
  23       if( dabs(db) .ge. dabs(da) .and. c .ne. 0.0d0 ) z = 1.0d0/c
  24    20 da = r
  25       db = z
  26       return
  27       end