官方文档有说明

Directories and ‘default_controller’, ‘404_override’
As you should know, the $route['default_controller'] and $route['404_override'] settings accept not only a controller name, but also controller/method pairs. However, a bug in the routing logic has made it possible for some users to use that as directory/controller instead.
As already said, this behavior was incidental and was never intended, nor documented. If you’ve relied on it, your application will break with CodeIgniter 3.0.

可见官方其实是把这当成一个BUG的，并不推荐这种做法。

不想显示index.php;应该是配置：config.phg

$config['index_page'] = ''; //去掉index.php

在根目录加入.htaccess

 RewriteEngine on
 RewriteCond %{REQUEST_FILENAME} !-f
 RewriteCond %{REQUEST_FILENAME} !-d
 RewriteCond $1 !^(index\.php|assets|robots\.txt)
 RewriteRule ^(.*)$ /index.php/$1 [L]

再者你的配置应该是 $route['default_controller'] ='index/home';

同问。我的也是将控制器分为前台和后台，index及admin。
为了实现默认控制器的访问，我在入口文件处index.php设置了

$routing['directory'] = 'index';//即默认访问前台控制器

其次在routes.php中设置了

$route['default_controller'] = 'home';//设置完毕就会直接访问index平台下的home控制器的index方法
问题：
现在要写后台，也就是admin的程序，请问怎么设置参数以及访问。