remove selected attribute of first selected element in select tag?

i have a multiple select tag, look at script please

<select multiple="multiple" size="5" id="cities_select">
     <option value="1">city1</option>
     <option value="2">city2</option>
     <option value="3">city3</option>
     <option value="4">city4</option>
     <option value="5">city5</option>
     <option value="6">city6</option>
     ................................
</select>

and jquery script:

$("#supply_cities_select").change(function()
{
    var i = 1;
    $('#supply_cities_select :selected').each(function(u) 
    {
         src += '&c'+i+'='+$(this).val();//generates the string like &c1=1&c2=2&c3=7...
         i++;
    });
})

i need my string not to have elements greater then 5

example:

if i allready have 5 selected elements, my string looks like

&c1=2&c2=3&c3=5&c4=6&c5=7

now if one more option will be selected, i need to get the string

 &c1=3&c2=5&c3=6&c4=7&c5=8

if be short, i need to remove selected attribute of first selected element.

(but i can't use .first here, because it can be element N8 the first selected)

how can i do it?

Thanks a lot.

UPDATE

var a = $("#supply_cities_select :selected").length;
if(a > 5)
{
     $("#supply_cities_select :selected:lt(1)").attr("selected",false);
}

it just remove th开发者_C百科e firs selected option, isn't it?

You can do this a little simpler usign .map() and :lt() like this:

var src;
$("#supply_cities_select").change(function() {
  src = $("#supply_cities_select :selected:lt(5)").map(function(i) {
      return '&c'+(i+1)+'='+this.value; //i starts at 0, so add 1
  }).get().join('');
})​;

You can try a demo here. The :lt() selector gets the first 5 (less than 5, 0-based, so (0-4), then we're using .map() to get the values into an array, then just calling .join() to get a string of that array added together.

For the update: to get the last 5 elements it's better to use .slice(), like this:

var src;
$("#supply_cities_select").change(function() {
  src = $("#supply_cities_select :selected").slice(-5).map(function(i) {
      return '&c'+(i+1)+'='+this.value; //i starts at 0, so add 1
  }).get().join('');
})​;

You can give it a try here.