C    IPNN(i) = I*4(NVMAX) = list of changing variable indices
C    NPNN    = I*4        = number of non-null parameters
C
      INTEGER*4 IPNN(NVMAX),NPNN
C
C Find number of non-null parameters and store their indices
C
      NPNN=0
      DO I1=1,NVEC
        IF (XVLO(I1) .LT. XVHI(I1)) THEN
          NPNN      =NPNN+1
          IPNN(NPNN)=I1
        ENDIF
      ENDDO
C
C Decide which variable to change
C
          INDX=IPNN(MAX0(1,
     +                   MIN0(1+IFIX(FLOAT(NPNN)*RAN(IFLAG)),
     +                        NPNN)
     +                  )
     +             )
C