Change array keys

mika79

Hi there!

I'm trying to link an array key to an object property, here's some sample code:

class Table {
  private $_columns = array();

  public function addColumn($c) {
    $this->_column[] = $c;
  }
  public function getColumn($index) {
    // ???
  }
}

class Column {
  public $Index;
.
.
.
}

What I am trying to achieve is to have a column index that is a Column object property. So when I say $mycolumn->Index=4; I mean be the 4th column inside the table. And when I say $mytable->getColumn(4); I mean give me the 4th column of $mytable.

I don't care the what are the column keys at table initialization(the $_columns keys), I'd like to explicitly define the column indexes for sort, save/load, ... and other purposes.

I believe there is no such thing as referenced array keys, where the array key would reference an index variable:

$key = 1;
$myarray[&key]="ABC";
$key = 4;
echo $myarray[4]; //returns ABC

Of course I could always iterate through $_columns array but I think this is not the right solution, too slow.

How would you solve this issue?

Thanks!

halojoy

hi
I think PHP 5 and PHP 6 is moving away from using references.
I never use them.

Writing code for portability into future PHP Versions
may use some smart alternative ways to referenced variables.

From http://docs.php.net/

---

allow_call_time_pass_reference Off/On

Whether to enable the ability to force arguments to be passed by reference at function call time.

This method is deprecated and
is likely to be unsupported in future versions of PHP/Zend.

The encouraged method of specifying which arguments should be passed by reference is in the function declaration.

You're encouraged to try and turn this option Off

and make sure your scripts work properly with it [/B]
..... in order to ensure they will work with future versions of the language

(you will receive a warning each time you use this feature, and the argument will be passed by value instead of by reference).
[B]Passing arguments by reference at function call time was deprecated for code cleanliness reason[/B]. 
Function can modify its argument in undocumented way if it didn't declared that the argument is passed by reference.
To prevent side-effects it's better to specify which arguments are passed by reference in function declaration only.
See also References Explained. [/QUOTE]Myself, I always use this setting
when developing:

allow_call_time_pass_reference Off

It is my simple way to make sure my scripts are for the future
and not for only for Todays PHP 5 and Yeasterdays PHP 4
... but for the soon to come PHP 6

regars ..to you 🙂
halojoy

laserlight

I think PHP 5 and PHP 6 is moving away from using references.
I never use them.

The word "reference" is overloaded (like "static", though the situation is not as bad). However, the "objects as references" concept aside, it is not true that PHP 5 and PHP 6 are moving away from using pass and return by reference, and in fact even call time pass by reference would not result in an E_NOTICE but an E_STRICT error in PHP 6 in view that "it was argued that there is nothing wrong with doing a call-time pass by reference". Nonetheless, I note that the reasoning was not given, and I would still recommend not using call time pass by reference since it can lead to unexpected behaviour for a given function implementation.

But we digress. I think that the solution is simple: use composition. The table owns the columns, and tracks them based on an associative array key. However, the client of the class will just access the columns by getters and setters.

mika79

Thanks for your note.

The thing is I'm not searching for a (&) reference solution, my question was : is it possible to make array keys work like references? As far as I know you can "change" an array key only by associating/copying the value to the new key and then unseting the old key (also array_flip can change keys but only for arrays).

$a=new Obj();
$b=$a;
$b->someMethod(); // working on one and only object, referenced by default

Can I somehow $_columns keys variable?

mika79

laserlight wrote:
But we digress. I think that the solution is simple: use composition. The table owns the columns, and tracks them based on an associative array key. However, the client of the class will just access the columns by getters and setters.

Can you post me some lines of code? How can I set a child object property and at the same time make this property the parent searching index?

laserlight

How can I set a child object property and at the same time make this property the parent searching index?

From what I see, there is no inheritance involved here, so it is not correct to talk about parent/child. We are talking about a container and its elements instead.

You want an element's property to be used as a key to that element. But you also want it such that changes to that element is reflected to that key. However, the element knows nothing about the container. If you want this to happen, to have to pass a reference (and here reference means "reference or pointer to the object") to the container to the element. Whether this is the correct decision depends on what are you trying to solve.

So, what are you really trying to model, and why do you need such behaviour? Why not just make the container control the keys, and thus relieve the element of the duty of maintaining its own index?

mika79

I'm using xml to store object relations and properties, here's a simplified example:

<table caption="My Table" query="SELECT name, second_name, address FROM contacts">
  <column title="Name" sqlcol="name" index="1"/>
  <column title="Address" sqlcol="address" index="3"/>
  <column title="Second Name" sqlcol="second_name" index="2"/>
</table>

The tag name represents my class, attributes are object properties, container key is incremented automatically after addChild is called:

$obj=$table->addChild(new $tagname());
//assign object properties

This function is used to create relations between many other instances, not only table <-> columns

You see? I fill the container (table) with columns and the table doesn't know anything about its elements. Furthermore I'm building an interface where a user can change the column position, column visibility,... all column properties. I'm able to sort elements by the index (using usort+special function that can compare objects by their properties), but can not reach an element by the index.

Weedpacket

While I admit to not being entirely clear on the situation (too much overloading of "reference", perhaps), I plead mental fatigue.

If you want to be able to ask the object for arbitrary properties (without them being predefined in the class definition), a get() method can capture any such requests and attempt to satisfy them in whichever way is appropriate. There is a corresponding set() method. This won't immediately work for numeric indices (since a property name can't start with a digit), but $obj->${'i'.$index} is legitimate (if ugly).

Another option is to equip the class with the methods specified by the [man]SPL[/man]'s ArrayObject interface. Then $obj[$index] would work.

But maybe it's something simpler:

public function getByIndex($i)
{
    return $this->${$this->columns[$i]};
}

So that if $foo->columns[3] = 'address', $foo->getByIndex(3) will return the value of $foo->address (assuming that such a property exists).

mika79

This is how am I doing it now, just to illustrate the case:

class Table {
  private $_columns = array();

  public function addColumn($c) {
    $this->_columns[] = $c;
  }
  public function getColumnByIndex($index) {
    // here's the ugly solution
    foreach($this->_columns as $c) {
      if($c->Index==$index) return $c;
    }
    return null;
  }
}

class Column {
  public $Index;
} 

$T=new Table();

//-------- XML columns data parsing ----------
// just add a new column, Table is not aware of column Indexes
$c1=new Column();
$c1->Index=1;
$T->addColumn($c1);

$c2=new Column();
$c2->Index=23;
$T->addColumn($c2);

$c3=new Column();
$c3->Index=9;
$T->addColumn($c3);
//------- end XML columns parsing --------

// give me the table column with the Index 23
$col=$T->getColumnByIndex(23);
echo $col->Index; // column OK

How can I get rid of this ugly solution? Is there some other way? Thanks!

Weedpacket

So each Column object has an Index property?

  public function addColumn($c) {
    $this->_columns[$c->Index] = $c;
  }

  public function getColumnByIndex($index) {
   if(isset($this->_columns[$index]))
     return $this->_columns[$index];
   return null;
  }

How's that?

Further, as I noted, you could implement ArrayObject; renaming getColumnByIndex to offsetGet and then you'd be able to write

echo $T[23];

In fact offhand I don't see why you couldn't pass integers or strings to offsetGet(), and have it decide whether you're asking for a column by index or by name.

Getting the public interface nice and tidy is a bit more important, methinks: that's the bit you're going to be using later on.

mika79

This is a nice trick

Weedpacket wrote:

  public function addColumn($c) {
    $this->_columns[$c->Index] = $c;
  }

but changing the column index property does not affect $this->_columns array index directly, so we're back at the beginning of this thread. Making an array index reference an object property I guess is not possible.