How to check if a library exist in a controller?

I understand that you want first to check if library loaded.
If not than you need to load, else you just use that library:

PHP Code:

public function some_other_method()
{
    if ( ! $this->is_lib($lib))
    {
        $this->load->library($lib);
    }
    // rest of your code
}

private function is_lib($lib)
{
    return is_object(@this->{$lib}) ?: FALSE;
}