Unable to check if a javascript checkbox array element is checked or not?

What is want is - when the checkbox is checked option no. 5 in select list will be selected and when the checkbox is unchecked option no. 0 will be selected in the select list.

The select list and the checkboxes are generated dynamically in the php code as below :

echo "<select name='coupons".$i."' id='coupons".$i."'>";
------- All Options --------
echo "</select>";


<input type='checkbox' name='myCheckbox[]' value='<?php echo $i."_".$row->id; ?>' onclick='setCCode("myCheckbox[]",<?php echo $i;?>)'> 

 -----------------------------------------------------------------------------

Solved the second requirement on my own now ..... thanks to all for your inputs

just added the following line in the checkAll() within the for loop

setCCode(children[i],i+1);

The javascript function :

function setCCode(checkbox_name,i) 
{   
    var form_object = document.getElementsByName(checkbox_name+"["+i+"]");
    var selname = document.getElementsByName("coupons"+i)[0];

    if(form_object.checked) {
            selname.selectedIndex = 5;
    }
    else {
        selname.selectedIndex = 0;
    } 
}

The above issue is solved....... thanks to all

Now what i need to do is when a user checks a checkbox to select or deselect all the dynamically generated checkboxes on the form the above logic should work.

<input type='checkbox' name='checkall' onChange="checkAll(this, 'myCheckbox[]')">
<span class="chkall">Check / Uncheck All</span>

<input type='checkbox' name='myCheckbox[]' value='<?php echo $i."_".$row->id; ?>' onclick='setCCode(this,<?php echo $i;?>)'>

The javascript code i am using to select/deselect all checkboxes on form is as below :

function checkAll(parent, field)
{
    var children = document.getElementsByName(field);
    var newValue = parent.checked;

    for (i = 0; i < children.length; i++){
    if (children[i].disabled == false) {    
        children[i].checked = newValue;
    }
    }
}

function setCCode(Sender,i) 
{   
  document.getElementsByName("coupons"+i)[0].selectedIndex = Sender.checked ? 5 : 0;
}