Counting number of ticked checkboxes

[SortedSites]

I have a form as below, I want to dynamically update the value of the text field "totalcost" depending on the number of checkboxes ticked by the user.

1 checkbox ==> totalcost = 48
2 checkboxes ==> totalcost = 79
3 checkboxes ==> totalcost = 115

Note there is a forth checkbox to indicate the acceptance of the terms and conditions, I don't want the checking of this checkbox to influence the count.

<form name="form1" id="form1" action="payment.asp" method="POST">
<input name="checkbox1" type="checkbox" id="checkbox1" value="Item 1" />
<input name="checkbox2" type="checkbox" id="checkbox2" value="Item 2" />
<input name="checkbox3" type="checkbox" id="checkbox3" value="Item 3" />

<input type="text" name="totalcost" value="" />

<input name="terms" type="checkbox" id="terms" value="accept" />

<input type="submit" name="Submit" value="Submit" />
</form>

Any ideas?

[Raffles]

Create an empty array.
Use document.getElementsByTagName('input') and loop through them, adding the ones which do not have the ID "terms" and which are of type "checkbox" to the array. Also attach "onclick" events to them, which call an update function.

In your update function, loop through the array you created earlier and count how many of them are currently checked. Then once the loop is over, update the cost based on what the count is.

[SortedSites]

Thanks, but I don't have problems with the algorithm or method, I have problems with syntax, my JavaScript skills are limited to the copy/paste variety. Not that I expect you to write the script for me, just wondering if anyone knows a script out there which is close enough that I can modify.

[SortedSites]

Something like this right?

function setValue(){ 
var total = 0;
	objs = document.form1.getElementsByTagName('input');
        for(i = 0; i<objs.length; i++){
		if(objs[i].type == 'checkbox' && objs[i].name != 'conditions'){
			if(objs[i].type == 'checkbox'){
                 total +=1 ;
		}
}
if(total == 1){
document.form1.totalcost.value = '48';
}

if(total == 2){
document.form1.totalcost.value = '79';
}
if(total == 3){
document.form1.totalcost.value = '115';
}


}

And I set the attribute onclick="javascript:setValue()" on each of the checkboxes.

Well I get an error "Object Expected". Whenever I click a checkbox

[Raffles]

window.onload = function() {
  var checks = document.getElementById('form1').getElementsByTagName('input'), items = [], totalcost = document.getElementById('totalcost');
  for (var i = 0; i < checks.length; i++) {
    if (checks[i].type != 'checkbox' || checks[i].id == 'terms') continue;
    items.push(checks[i]);
    checks[i].onclick = function() {
      var total = 0;
      for (var k = 0; k < items.length; k++) {
        if (items[k].checked) total++;
      }
      if (total == 0) totalcost.value = '48';
      else if (total == 1) totalcost.value = '48';
      else if (total == 2) totalcost.value = '79';
      else if (total == 3) totalcost.value = '115';
    }
  }
}

No need to add "onclick" anywhere. You do need to add the ID "totalcost" to the totalcost text input.

[SortedSites]

Wow thanks works brilliantly.