#codingexercise
Int GetCountDuplicatesSum (int [] A)
{
if (A == null) return 0;
return A.CountDuplicatesSum();
}

Today I want to discuss PHP programming language as quickly as possible. Its useful in the Model View Controller or MVC which is a widely used pattern for software applications.
The basic syntax involves
<?php
echo 'Hello World'
...
The types involved are boolean, integer, float, string, array and object.  The last two are referred to as compound types.
eg,
$a_bool = TRUE;
$a_str = "foo";
$an_int = 12;
$array = [
    "foo" => "bar",
    "bar" => "foo"
];
$b = array_values($array)
?>
There are two special types resource and NULL.
The resource type is a special variable, holding a reference to an external resource and its type is got by get_resource_type(). A resource variable holds special handlers to opened files, database connections and images. Resources are automatically collected.

mixed, number and callback are also used. These are called pseudo -types
mixed indicates that a parameter may accept multiple types,
number indicates that a parameter can be either integer or float.
callback and callable are used to denote functions as parameters and usually passed by its name as string.

classes are defined simply as :
<? php
class MyClass{
  public $property = "Hello World";
  public function Call()
  {
     call_user_func(array($this, 'Callback'));
  }

  public function Callback()
  {
     echo $this->property;
  }
}
$m = new MyClass;
$m->call();
>
keywords such as interface, implements and extends are available. Scope resolution is with :: and objects can be cloned with the _clone method.

PHP does not require explicit type definition in variable declaration. The addition operator is a good example.
$ foo = "8"
$foo += 2
$foo  += 1.3
$foo = 5 + "10 some_string_ignored"
Typecasting and strict === are checked.
Variables can have local or static scope
Data from get or post requests are found as follows:
echo $_POST['username'];
echo $_REQUEST['username'];
we also get $_COOKIE['count']

To make an MVC application we use say CakePHP framework.

We clone it from git at git://github.com/cakephp/cakephp.git

and the MVC application has a folder structure like this:
/path_to_document_root
    /app
    /lib
    /plugins
    /vendors
    .htaccess
    index.php
    README

we may have to give folder permissions with
<?php echo exec('whoami'); ?>
chown -R www-data app/tmp
A database connection is simply a configuration
public $default = array(
    'datasource' => 'Database/Mysql',
    'persistent' => false,
    'host' => 'localhost',
    'port' => '',
    'login' => 'mylogin',
    'password' => 'mypassword',
    'database' => 'location-volume-share',
    'schema' => '',
    'prefix' => '',
    'encoding' => 'utf8'
);
Controllers can be defined by
// File: /app/Controller/ShareController.php
class shareController extends AppController {
    public $helpers = array('Html', 'Form');
    public function index() {
         $this->set('posts', $this->Post->find('all'));
    }
    public function edit($id = null) {
        if (!$id) {
            throw new NotFoundException(__('Invalid post'));
        }
        $params = explode(':', $this->request->data, 3)
        $location = $params[0]
        $volume = $params[1]
        $share = $params[2]
        $this->set($id, $location, $volume, $share);
    }
}

Now we will continue our discussion on preconditioning in conjugate gradients. We saw that we could use transformation to make the iterations quicker. The effectiveness of a preconditioner is determined by the condition number of M inverse applied to A as discussed earlier and occassionally by clustering of its eigenvalues. The simplest preconditioner is a diagonal matrix whose diagonal entries are identical to those of A.