如何在codeigniter中获取值数组？

Question

How to get value post array in codeigniter? I have problem when I get value post array and echo the value. How to show post value when submit? here the error message:

A PHP Error was encountered

Severity: Notice

Message: Uninitialized string offset: 0

Filename: controllers/blablabla

view html:

<?php $i=0; foreach ($doc as $row) { ?>
<label>
<input name="size[<?php echo $i; ?>]" type="checkbox" value="<?php echo $row['doc']; ?>">&nbsp;&nbsp;<?php echo $row['doc']; ?>
</label>
<?php $i++; } ?>

controller :

$size = $this->input->post('size');
for ($i=0; $i<count($doc); $i++) 
{
   echo $size[$i];
}