How to get value post array in codeigniter?

Change the way name of checkbox written as follows,

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

And in post method,

$size_arr = $this->input->post('size');foreach($size_arr as $v){  echo $v;}

if for some reason it is not working then check with,

$size_arr = $_POST['size'];foreach($size_arr as $v){  echo $v;}

EDIT

One more alternative,

$arr = $this->input->post();$size_arr = $arr['size'];foreach($size_arr as $v){  echo $v;}

Core version,

$arr = $_POST;$size_arr = $arr['size'];foreach($size_arr as $v){  echo $v;}

Your html form code should be like below.

<input name="size[<?php echo $i; ?>]" type="checkbox" value="<?php echo $row['doc']; ?>"> 

Inside controller your code should be like below.

$size = $this->input->post('size');foreach($size as $sa)  {   echo $sa;}

No need to use $i in checkbox name in view file just take an array

View file

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

Controller

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