Pass values from a checkbox in an array populate the unchecked with some value

Question

I’m trying to pass values checkbox in a array, but the unchecked fields did not populate the array. I need to fill in the unchecked fields 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 as output:

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

I need to popular the unchecked 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"}