change the select option selection

I want to change the select option sel开发者_开发问答ected to another value. I.e.,

 <select id="myid"> 
<option value="1" selected="selected">1 </option>
<option value="2" >2 </option>
<option value="3" >3 </option>
<option value="4" >4 </option>
</select>

I have to trigger a change in this select and select the 3rd option I have the value how can I do it.?

I am adding an option each time by a function and I want after each addition of the option, I want it to be selected to do further activities.

To change by index

<input type="button" value="Change Select" onclick="changeSelect(3)" />

function changeSelect(x) {
    document.frm.myid.selectedIndex = x-1;
}

to change by value

<input type="button" value="Change Select" onclick="changeSelect1(3)" />

function changeSelect1(x) {
  var myid = document.getElementById('myid');
  var st = myid.options;
  for(var i=0; i<st.length; i++) {
    if(st[i].value == x) {
      myid.selectedIndex = i;
    }
  }
}

function Change() {
            for (i = 0; i < document.getElementById('ddStream').options.length; i++) {
                if (document.getElementById('ddStream').options[i].value == 1)
                document.getElementById('ddStream').selectedIndex = i;
            }
            alert(document.getElementById('ddStream').selectedIndex);
            return false;
        }

<select id="ddStream" onchange="return Change()">
                            <option value="">EEE</option>
                            <option value="">ECE</option>
                            <option value="">CSE</option>
                            <option value="1">E&I</option>
                        </select>

This is the basic syntax.. The other things that you have mentioned will go by the logic u write.. Eg, to select the last option that u add dynamically, document.getElementById('ddStream').selectedIndex = document.getElementById('ddStream').options.length -1

selected="selected" selects the option you want, just put it on the 3rd option tag

E.g.

<select id="myid"> 
  <option value="1">1 </option>
  <option value="2">2 </option>
  <option value="3" selected="selected">3 </option>
  <option value="4">4 </option>
</select>