SUBROUTINE locate(xx,n,x,j,nxdim)
c     =================================
c
      IMPLICIT REAL*8(A-H,O-Z)
      dimension xx(nxdim)
c
      jl=0
      ju=n+1
10    if(ju-jl.gt.1)then
        jm=(ju+jl)/2
        if((xx(n).ge.xx(1)).eqv.(x.ge.xx(jm)))then
          jl=jm
        else
          ju=jm
        endif
      goto 10
      endif
      if(x.eq.xx(1)) then
        j=1
      else if(x.eq.xx(n)) then
        j=n-1
      else
        j=jl
      endif
      return
      END