Filling values in a table

DECLARE
   cnt NUMBER(8):=0;
   myrow ROWID:=NULL;
 
   CURSOR mycursor IS
     SELECT ROWID 
     FROM MYTABLE 
     WHERE new_column is NULL
        AND  ROWNUM=1;
BEGIN
     LOOP
           OPEN mycursor;
           FETCH mycursor INTO myrow;
           EXIT WHEN mycursor%NOTFOUND;
           CLOSE mycursor;
           UPDATE mytable
                 SET new_column=cnt 
                 WHERE ROWID=myrow;
            cnt:=cnt+1;
     END LOOP;
     CLOSE mycursor;
     COMMIT;
END;
/