How can I get a form variable if the name is a parameter?

Hello,
I have the following form that contains various parameters P1..... P10. I only give the example for P1 and P2:

<form onSubmit="return false" method="post" name="parameters" action="">
<table border=0>
<tr>
<td>P1:</td>
<%
a vector v will contain the correct list
%>
<td><select name="p1"  onChange="getSelect(this)">  
<%
    for (int k=0;k<v.size();k++)
    {
%>
      <option value="<%=v.elementAt(k)%>"><%=v.elementAt(k)%></option>
<%
    }
%>
</select></td>
<td>
<input type="button" onClick="reDirect('p1')" value="Modify"></td>
<tr>
<tr>
<td>P2:</td>
<%
The vector v will contain the correct list
%>
<td><select name="p2"  onChange="getSelect(this)">  
<%
    for (int k=0;k<v.size();k++)
    {
%>
      <option value="<%=v.elementAt(k)%>"><%=v.elementAt(k)%></option>
<%
    }
%>
</select></td>
<td>
<input type="button" onClick="reDirect('p2')" value="Modify"></td>
</table>
</form>

Code (markup):

Now, inside the javascript function reDirect(fieldName) I would like to get the content of the fieldName as follows:

function reDirect(fieldName)
{
.....
fieldName=document.parameters.p1.value; or
fieldName = document.parameters.p2.value 
but with only one line and having fieldName instead of "p1" or "p2".
}

Code (markup):

In other words, I would like to find a way of getting the parameter value doing:
document.parameters.<fieldName>.value.

Is there a way of doing it?

THank you for your help.

 

Problem solved with getElementbyId.