Form Validation For Checkbox Named As Array

I have a number of checkboxes and radio buttons in an HTML form that are named as arrays, ie.

<INPUT type="checkbox" ID="i1" VALUE="MILK" NAME="Q1[]">
<INPUT type="checkbox" ID="i2" VALUE="SUGAR" NAME="Q1[]">
<INPUT type="checkbox" ID="i3" VALUE="BISCUIT" NAME="Q1[]">

I need to keep the 'NAME' as an array as above because of the form processing that is used after submit.

I was really hoping to use Javascript to validate the form and make sure that each checkbox group has at least one option selected , but my javascript messes up because the 'NAME' is Q1[] as opposed to Q1. I had been using:

<script>
function validate() {
if (!(mainform.Q1[0].checked || mainform.Q1[1].checked|| mainform.Q1[2].checked)) {
alert('Please answer question 1.');
event.returnValue=false;
}}
</script>

but this only works when I name the checkboxes NAME="Q1". Is anyone aware of how to get the validate function to work (or another way of validating the form) even when the checkboxes and radio buttons are named like NAME="Q1[]".