validate atleast one check box is checked ....

validate atleast one check box is checked ....

I have 4 check boxes
that are dynamically generated 

  1. <table width="100%" border="0" cellspacing="0" cellpadding="0">
  2.           <tbody>
  3.             <tr>
  4.               <td width="26%"><input name="membercategory[]" type="checkbox" id="membercategory[]" value="1" />
  5.                 Life Membership </td>
  6.               <td width="26%"><input name="membercategory[]" type="checkbox" id="membercategory[]" value="2" />
  7.                 Donor Member </td>
  8.             </tr>
  9.             <tr>
  10.               <td width="26%"><input name="membercategory[]" type="checkbox" id="membercategory[]" value="3" />
  11.                 Well-Wisher </td>
  12.               <td width="26%"><input name="membercategory[]" type="checkbox" id="membercategory[]" value="4" />
  13.                 Petron Member </td>
  14.             </tr>
  15.           </tbody>
  16.         </table>

I wish to validate  at least one check box is clicked

for single checkbox
I am using following following javascript code .............. the checkbox name is 'terms'

  1. if (!document.form1.terms.checked)
  2.   {
  3.   alertMsg += " - " + "Please Accept Terms and conditions  " + "\n";
  4.   }
Since number of check boxes are dynamic I cannot use and/or conditions

pls help