How To Uncheck A Check Box

How does one uncheck a check Box with Javacript when the rows of the Table are dynamically created byPhp so we do not know what the row numbers are going to be before hand.That is we have these 2 check boxes:

<input type="checkbox" name="approve[]" value="<?php echo $user_id; ?>" checked onClick="uncheck(reject, <?php echo ($row - 1); ?>>
&
<input type="checkbox" name="reject[]" value="<?php echo $user_id; ?>" onClick="uncheck(approve, <?php echo ($row - 1); ?>">

So if they click on check box "reject", we then want check box "approve" unchecked and vis-versa.FYI, I was using this Javascript code:

function uncheck(field, i)
{
field[i].checked = false ;
}

But the combination of above is not working!