Enable/disable of textbox on option selected from drop down menu

What I want is that the text box is only accessible if a certain option is picked from the drop down menu and I have an html form as below:

 <tr>
 <td>a.&nbsp;Did any  of your staff participate in training or orientation sessions related to any aspect of social performance management, during the reporting year? </td>
 <td >
 <p>
   <select name="mfi_4_a_i">
     <option>Yes</option>
     <option>No</option>
     <option>No, but planning in future</option>
   </select>
 </p>
 <p>if not,and not planning please explain why not </p>
 <input type="text" name="mfi_4_a_ii" id="sdd" />
 </tr>

Now when a user selects the option No, but planning in future then the text box must be enabled otherwise the textbox must be disabled.

How do I accomplish开发者_运维知识库 this?

You should call the javascript function for this.

<select id="mfi_4_a_i" name="mfi_4_a_i" onChange="changetextbox();">
    <option>Yes</option>
    <option>No</option>
    <option>No, but planning in future</option>
</select>

<input type="text" name="mfi_4_a_ii" id="sdd" />

<script type="text/javascript">
function changetextbox()
{
    if (document.getElementById("mfi_4_a_i").value === "noy") {
        document.getElementById("sdd").disable='true';
    } else {
        document.getElementById("sdd").disable='false';
    }
}
</script>

For me the document.getElementById("sdd").disabled='false' did not work so I used

document.getElementById("sdd").disabled='';

if (document.getElementById("mfi_4_a_i").value === "noy") {
    document.getElementById("sdd").disabled='true';
} else {
    document.getElementById("sdd").disabled='';
}

Just corrections... removal of ; , === , noy , 'true' #no quotes

onChange="changetextbox();"
if (document.getElementById("mfi_4_a_i").value == "no") {
document.getElementById("sdd").disabled=true;

Without doing a roundtrip to the server, you'll have to do that with javascript

document.getElementById("the_id").disabled=''; works for me. works, much like style.display