function chooseFilm(){
   clean();   
   document.forms[0].film_.value = document.forms[0].film.value;
   if(document.forms[0].film.value == ""){
      alert("please choose a film first!");
      clean();
      document.forms[0].day.disabled = true;
      document.forms[0].time.disabled = true;
      return false;
   }
   chooseDay();
}
function chooseDay(){
   if(document.forms[0].film_.value == ""){//should not happened!
      alert("no film has choosen!");
      return false;
   }
   document.forms[0].day.disabled = false;
   var filmid = document.forms[0].film_.value;
   var day_array = new Array();
   for(i=0;i<showid_filmid_day_array.length;i++){
      if(showid_filmid_day_array[i][1] == filmid){
         day_array.push(showid_filmid_day_array[i]);
      }
   }
   //construct day select tag
   var day_cache_array = new Array();

   for(i=0;i<day_array.length;i++){
      var showid = day_array[i][0];
      var filmid = day_array[i][1];
      var day = day_array[i][2];
      if(day_cache_array.indexOf(day_array[i][2]) == -1 ){
         day_cache_array.push(day_array[i][2]);   
      }   
   }//for
   var ttt = document.forms[0].day.options.length;
   for(i=0;i<ttt;i++){
      document.forms[0].day.options[i] = null;
   }
   for(i=0;i<day_cache_array.length;i++){
      document.forms[0].day.options[i] =
         new Option(day_cache_array[i], day_cache_array[i]);
   }
   chooseTime();
}
function chooseTime(){
   if(document.forms[0].film_.value == ""){//should not happened!
      alert("no film has choosen!");
      return false;
   }
   document.forms[0].time.disabled = false;
   var filmid = document.forms[0].film_.value;
   var time_array = new Array();
   for(i=0;i<showid_filmid_time_array.length;i++){
      if(showid_filmid_time_array[i][1] == filmid){
         time_array.push(showid_filmid_time_array[i]);
      }
   }

   //construct day select tag
   var time_cache_array = new Array();
   for(i=0;i<time_array.length;i++){
      var showid = time_array[i][0];
      var filmid = time_array[i][1];
      var time = time_array[i][2];
      if(time_cache_array.indexOf(time) == -1 ){
         time_cache_array.push(time);   
      }   
   }//for

   var time_length = document.forms[0].time.length;
   while(time_length>0){
      document.forms[0].time.options[time_length] = null;
      time_length--;
   }
   for(i=0;i<time_cache_array.length;i++){
      document.forms[0].time.options[i] =
         new Option(time_cache_array[i], time_cache_array[i]);
   }
}
function validate(){
   if( document.forms[0].film.value == "" ){
      alert("please choose film name!");
      return false;   
   }
   if(document.forms[0].day.value == ""){
      alert("please choose a day!");
      return false;   
   }
   if(document.forms[0].time.value == ""){
      alert("please choose time!");
      return false;   
   }
   document.forms[0].submit();
}