Passing values from a checkbox to an array populating those not checked with some value

I'm trying to pass checkbox values to a array , but unchecked fields do not populate array . I need to fill in the fields not checked with the value "N" for example:

Example in a loop of 5 results:

<?php
while(5x) : ?>

    <td><input  name="ocultarsub[]" type="checkbox" value="S" <?php  if($ocultar_sub =='S'){echo "checked"; }  ?> ></td>

<?php 
endwhile; 
?>  

I get output:

array(2) { [0]=> string(1) "S" [1]=> string(1) "S" }

I need to popular those not checked with the letter N for example:

array(5) { [0]=> string(1) "S" [1]=> string(1) "S" [2]=> string(1) "N" [3]=> string(1) "N" [4]=> string(1) "N"}