Question

Using PHP 5.3.10, I created a link-list class and am trying to save a list of football players.

After calling the add function, it seems that the object never retains any information. var_dump($playerList) returns NULL for both my head and tail pointers. Or, if I replace it with var_dump($playerList->count), it prints nothing no matter where I place the var_dump count statement.

I have been through the manual and cannot find the error in my syntax. My gut is telling me mysql_fetch_array is doing something funky. As stated below, my testing shows that values are in fact being passed around when I call playerList->add(). Anyhow, here is my simple code:

/* Populates lists with available players. */
function populateList($sql)
{
    $playerList = new PlayerList();
    while ($row = mysql_fetch_array($sql, MYSQL_NUM)) 
    {
        $playerList->add(new Player($row[0], $row[1], $row[2], $row[3], $row[4]));
    }
    var_dump($playerList);
}

And my linked list class:

include 'PlayerNode.php';

class PlayerList
{
    public $head;
    public $tail;
    public $count;

    function PlayerList()
    {
        $head = null;
        $tail = null;
        $count = 0;
    }

    function add($player)
    {
        $count ++;            
        $node = new PlayerNode($player);

        //First time in
        if ($head == null)
        {
            $head = $node;
            $tail = $node;
            $head->nextPtr = null;
        }
        // All other times
        else
        {
            $tail->nextPtr = $node;
            $tail = $node;
            $node->nextPtr = null;
        }
        $count++;
    }               
}

I can place var_dump($node) and echo statements in the linked list class and observe that PlayerNode is working correctly.

But, another strange observation… if($head==null) ALWAYS evaluates to true too. Could this be related?

Answer 1

Insertion in the head of the Singly Linked Lists :
We can easily insert the elements in the head of the list. So how we do it? Create a new node, set the next of the new node point to the current head node, and set the head variable (in the class) point to the new node. This method works even if the Linked List is empty. Note that we set the next of the new node point to the head node, before we sent the head variable to point to the new node.

Insertion in the tail of the Singly Linked Lists:
We can also easily insert elements in the tail of the Linked List, provided we keep a reference for the tail node of the Linked Lists. Create an new node set the next of the new node to null, set the next of the tail node point to the new node, set the tail variable to point to the new element. Note we set the next of the previous tail node before we change the tail variable to point to the new node.

In all the other times add the new node to the head or tail.

              // All other times if head
                else{
                        $temp = $head;
                        $head = $node;
                        $node->nextPtr = $temp;
                        count ++; 
                    }