java脚本实现下拉框和文本框的数据交换

          

   <script language="javascript">...
                              function change(va)
                              ...{
                                   if(va.value==<%=a%>)
                                      str=' <input name="cnum" type="text" id="cnum" value="'+va.value+'" size="15" ><br><input name="course" type="text" size="15" value="填写新课程名" >';
                                    else
                                    str=' <input name="cnum" type="text" id="cnum" value="'+va.value+'" size="15" > <input type="hidden" name="course" value=" ">';
                                    
                                    window.show.innerHTML=str;
                              }
                              </script>  
<table>
                           <tr>
                              <td width="48%" align="center">课程号</td>
                              <td width="52%" id="show">     </td>
                            </tr>
</table>

 <select name="select4" onChange="change(this)">
                                <option   selected>选择课程</option>
                                <option value="<%=a%>" >添加新课</option>
                             
                                  <%... for (int i = 0; i < length; i++){ %>
                                   <option  value="<%=titlelist3[i]%>" ><%=titlelist[i]%></option>
                             <%}%>
                                    </select>