jquery - Set value with dynamic attributes -

Why does this script not work? I want to give value to parent_id in the input, please tell me what to do

     
 
 
 You have  post_id = "1"  and  Id = "parent_id_2" , you can not see the mismatch.  
 It should be  
  $ ("# parent_id _" + pi) .val (id);