function removeOptions(select_box){
	var i;
	select_box.options.length = 0;
}

function addOption(select_box,text,value,selected) {
  var optn = document.createElement("OPTION");
  optn.text = text;
  optn.value = value;
  if (selected) {
  	optn.selected = true;
  }
  select_box.options.add(optn);
}

function update_sub_categories(selected_id) {
	var select_box = document.matters_form.sub_category_F;
	var selected = document.matters_form.category_F.selectedIndex;
	if(selected == -1) {
		return false;
	}
	removeOptions(select_box);
	var i = 0;
	while(i < subcategoriesArr[selected].length) {
		if (selected_id == -1) {
  		if (i == 0) {
  			addOption(select_box, subcategoriesArr[selected][i + 1], subcategoriesArr[selected][i], true);
  		} else {
  			addOption(select_box, subcategoriesArr[selected][i + 1], subcategoriesArr[selected][i], false);
  		}
		} else {
			if (selected_id == subcategoriesArr[selected][i]) {
  			addOption(select_box, subcategoriesArr[selected][i + 1], subcategoriesArr[selected][i], true);
  		} else {
  			addOption(select_box, subcategoriesArr[selected][i + 1], subcategoriesArr[selected][i], false);
  		}
		}
		i += 2;
	}
}