How to retain the dropdown value after submitting the form

I had this question after viewing Retain the value of Drop down box in jsp.
I tried the below code but i'm getting error : alloc_Type is undefined on refresh.
<%
 String riskTypeValue = request.getParameter("riskType");
if(riskTypeValue == null)riskTypeValue = "0";

String allocTypeValue = request.getParameter("allocType");
if(allocTypeValue == null)allocTypeValue = "0";
%>

<script>
function retainValue(){
      //alert("retainValue "+alloc_Type +" && " +risk_Type);
    var element1 = document.forms[0].allocType;
    for(x = 0; x < element1.length; x++){
            if(element1.options[x].value == "<%=alloc_Type%>")
            {
                  element1.options[x].selected = true;
                  break;
            }
    }
    var element2 = document.forms[0].riskType;
    for(x = 0; x < element2.length; x++){
            if(element2.options[x].value == "<%=risk_Type%>"){
                  element2.options[x].selected = true;
                  break;
            }
    }
}
</script>


<tr>
                        <td>&nbsp;</td>
                        <td class="smalltext" width="150px"><b>Allocation Type:</b></td>
                        <td class="smalltext">
                               <select id="allocType"
                                          class="filter_smalltext" name="allocType">
                                    <c:forEach items="${dataBO.alloc_types}" var="allocTypes">
                                          <option value="${allocTypes.key}" ${ allocTypes.key eq dataBO.alloc_type ? 'selected':'' }>
                                                <c:out value="${allocTypes.value}"></c:out>
                                          </option>
                                    </c:forEach>
                              </select>
                        </td>
                        <td>&nbsp;</td>
                  </tr>