Database Error 1064 while executing a SELECT query in CodeIgniter

I am trying to code CodeIgniter authentication, but when I try, I make an error like this

A Database Error Occurred

Error Number: 1046

No database selected

SELECT * FROM (user) WHERE user_username = 'amanda' AND user_passwordMD5("12345") =

Filename: C:\xampp\htdocs\codeigniter\system\database\DB_driver.php

Line Number: 330

My database.php file:

$db['default']['hostname'] = 'localhost';
$db['default']['username'] = '';
$db['default']['password'] = '';
$db['default']['database'] = 'authtentication';
$db['default']['dbdriver'] = 'mysql';
$db['default']['dbprefix'] = '';
$db['default']['pconnect'] = TRUE;
$db['default']['db_debug'] = TRUE;
$db['default']['cache_on'] = FALSE;
$db['default']['cachedir'] = '';
$db['default']['char_set'] = 'utf8';
$db['default']['dbcollat'] = 'utf8_general_ci';
$db['default']['swap_pre'] = '';
$db['default']['autoinit'] = TRUE;
$db['default']['stricton'] = FALSE;

Check the database.php file in application/config/

$db['default']['hostname'] = 'localhost';
$db['default']['username'] = 'root';
$db['default']['password'] = 'mypassword';
$db['default']['database'] = 'my_database'; // here is the database selection