php - Notice: Undefined offset: 0 in -

i getting php error, mean?

notice: undefined offset: 0 in  c:\xampp\htdocs\mywebsite\reddit_vote_tut\src\votes.php on line 41 

from code:

<?php  include("config.php");   function getallvotes($id)  {      $votes = array();      $q = "select * entries id = $id";      $r = mysql_query($q);      if(mysql_num_rows($r)==1)//id found in table      {          $row = mysql_fetch_assoc($r);          $votes[0] = $row['votes_up'];          $votes[1] = $row['votes_down'];      }      return $votes;  }   function geteffectivevotes($id)  {          $votes = getallvotes($id);          $effectivevote = $votes[0] - $votes[1];    //error thrown here         return $effectivevote;  }   $id = $_post['id'];  $action = $_post['action'];   //get current votes  $cur_votes = getallvotes($id);   //ok, update votes   if($action=='vote_up') //voting  {       $votes_up = $cur_votes[0]+1;     //and error thrown here       $q = "update threads set votes_up = $votes_up id = $id";  }  elseif($action=='vote_down') {      $votes_down = $cur_votes[1]+1;      $q = "update threads set votes_down = $votes_down id = $id";  }   $r = mysql_query($q);  if($r) {      $effectivevote = geteffectivevotes($id);      echo $effectivevote." votes";  }  elseif(!$r) //voting failed  {      echo "failed!";  }  ?> 

you asking value @ key 0 of $votes. array not contain key.

the array $votes not set, when php trying access key 0 of array, encounters undefined offset [0] , [1] , throws error.

if have array:

$new_array = array('1','2','3'); 

we can access:

$new_array[0]; $new_array[1]; $new_array[2]; 

if try , access:

$new_array[3]; 

we error "notice: undefined offset: 0"