OOP / scope problem

LiseA

Hejsa!

Already asked this on the newbie forum, without getting much help. Maybe you can help?

I expect this code to produce:

8 9
Hej verden

but it actually produces

8
Hej verden

I suspect I have a scope problem in this line:

$this->tail->next = new IntNode($el,null);

Something about the left side having 3 parts...

Anyway. Help?

Lise

IntNode.obj

<?php
// a node in an integer singly linked list class

class IntNode {
        var $info; // integer
        var $next; // IntNode
        function IntNode($i,$n) {
                $this->info = $i;
                $this->next = $n;
        }
}

?>

IntSLList.obj

<?php
        // singly linked list class to store integers

    class IntSSList {
            var $head; // IntNode
            var $tail; // IntNode
            function IntSLList() {
                    $this->head = null;
                    $this->tail = null;
            }
            function isEmpty() {
                    return ($this->head == null);
            }
            function addToHead($el) {
                    $this->head = new IntNode($el,$this->head);
                    if ($this->tail == null) {
                            $this->tail = $this->head;
                    }
            }
            function addToTail($el) {
                    if (!$this->isEmpty()) {
                            $this->tail->next = new IntNode($el,null);
                            $this->tail = $this->tail->next;
                    } else {
                            $this->tail = new IntNode($el,null);
                            $this->head = $this->tail;
                    }
            }
            function printAll() {
                    $tmp = $this->head;
                    while ($tmp != null) {
                            print $tmp->info;
                            $tmp = $tmp->next;
                    }
                    print "<br>\n";
            }
    }
?>

test.php

<?php
        include "IntNode.obj";
        include "IntSLList.obj";

    $test3 = new IntSSList();
    $test3->addToTail(8);
    $test3->addToTail(9);
    $test3->printAll();
?>
<html><head><title>Hej verden</title></head>
<body>
Hej verden
</body>
</html>

tomhath

The problem is that the "$next" variable contains a copy of the object, not a pointer to the object. So when you make this assignment: $this->head = $this->tail; you now have two copies of the node. You then create a new object and store it in $this->tail, but $this->head still contains a copy of the first object you created.

In other words, you never created a linked list. All you have is two variables (head and tail) each of which contains a copy of a single object.

tomhath

Try writing your addToTail function this way, using references to the nodes instead of copies
(note the '&' $this->tail = &$this->head ) 😃

http://www.phpbuilder.com/manual/language.variables.php

                function addToTail($el) {
                        if (!$this->isEmpty()) {
                                $this->tail->next = new IntNode($el,null);
                                $this->tail = &$this->tail->next;
                        } else {
                                $this->head = new IntNode($el,null);
                                $this->tail = &$this->head;
                        }
                }

LiseA

Hejsa!

Thank you! This indeed solved the problem.

Lise

LiseA

Hejsa!

Thought I had my problems solved - but then apparently not...

I expect my new code to produce:

It is empty
5 and tail is: 5
4 5 and tail is: 5
4 5 6 and tail is: 6
4 5 6 7 and tail is: 7
6 and tail is: 6
6 7 and tail is: 7
5 6 7 and tail is: 7
4 5 6 7 and tail is: 7
Hello world

But it actually produces:

It is empty
5 and tail is: 5
4 5 and tail is: 4
4 6 and tail is: 6
4 6 7 and tail is: 7
6 and tail is: 6
6 7 and tail is: 7
5 6 7 and tail is: 7
4 5 6 7 and tail is: 7
Hello world

I suspect either addToHead or IntNode() is wrong.

This is my new code:

<?php
// a node in an integer singly linked list class

class IntNode {
        var $info; // integer
        var $next; // IntNode
        function IntNode($i,$n) {
                $this->info = $i;
                $this->next = $n;
        }
}

?>

<?php
        // singly linked list class to store integers

    class IntSLList {
            var $head; // IntNode
            var $tail; // IntNode
            function IntSLList() {
                    $this->head = null;
                    $this->tail = null;
            }
            function isEmpty() {
                    return ($this->head == null);
            }
            function addToHead($el) {
                    $this->head = new IntNode($el,$this->head);
                    if ($this->tail == null) {
                            $this->tail = &$this->head;
                    }
            }
            function addToTail($el) {
                    if (!$this->isEmpty()) {
                            $this->tail->next = new IntNode($el,null);
                            $this->tail = &$this->tail->next;
                    } else {
                            $this->tail = new IntNode($el,null);
                            $this->head = &$this->tail;
                    }
            }
            function printAll() {
                    $tmp = $this->head;
                    while ($tmp != null) {
                            print "$tmp->info ";
                            $tmp = $tmp->next;
                    }
print "and tail is: " . $this->tail->info;
                        print "<br>\n";
                }
        }
?>

<?php
        include "IntNode.obj";
        include "IntSLList.obj";

    $test1 = new IntNode(2,null);
    $test2 = new IntNode(3,$test1);

    $test3 = new IntSLList();
    if ($test3->isEmpty()) {
            print "It is empty<br />";
    } else {
            print "There's something here<br />";
    }
    $test3->addToHead(5);
    $test3->printAll();
    $test3->addToHead(4);
    $test3->printAll();
    $test3->addToTail(6);
    $test3->printAll();
    $test3->addToTail(7);
    $test3->printAll();

    $test3 = new IntSLList();
    $test3->addToTail(6);
    $test3->printAll();
    $test3->addToTail(7);
    $test3->printAll();
    $test3->addToHead(5);
    $test3->printAll();
    $test3->addToHead(4);
    $test3->printAll();
?>
<html><head><title>Hello world</title></head>
<body>
Hello world
</body>
</html>

tomhath

I haven't tested this suggestion, but I suspect you need to pass a reference to the head of the list, not a copy of the list ( &$this->head ) :

function addToHead($el) {
                        $this->head = new IntNode($el,  &$this->head );
                        if ($this->tail == null) {
                                $this->tail = &$this->head;
                        }

LiseA

Hejsa!

What you suggest might be a step in the right direction. But my program still produces the same, wrong result.

Lise

tomhath

Very confusing. This seems to be working:

<?php
  // a node in an integer singly linked list class

  class IntNode {
    var $info; // integer
    var $next; // IntNode

function IntNode($i, $n) {
  $this->info = $i;
  $this->next = &$n;
}
  }

   // singly linked list class to store integers

  class IntSLList {
    var $head; // IntNode
    var $tail; // IntNode

function IntSLList() {
  $this->head = null;
  $this->tail = null;
}

function isEmpty() {
  return ($this->head == null);
}

function addToHead($el) {
   $newhead = new IntNode($el, &$this->head);
   $this->head = &$newhead;

   if ($this->tail == null) {
      $this->tail = &$newhead;
   }
}

function addToTail($el) {

  if (!$this->isEmpty()) {
    $this->tail->next = new IntNode($el,null);
    $this->tail       = &$this->tail->next;
  } else {
    $this->head = new IntNode($el,null);
    $this->tail = &$this->head;
  }
}

function printAll() {

  $tmp = $this->head;
  echo "<P>";
  while ($tmp != null) {
    print "$tmp->info ";
    $tmp = $tmp->next;
  }

  print " and tail is: " . $this->tail->info;
  print "<p><p>\n";
}
  }

  $test3 = new IntSLList();

  $test3->addToHead(5);
  $test3->printAll();

  $test3->addToHead(4);
  $test3->printAll();

  $test3->addToTail(6);
  $test3->printAll();

  $test3->addToTail(7);
  $test3->printAll();

  $test3->addToHead(3);
  $test3->printAll();
 ?>

LiseA

Hejsa!

Implemented your suggestion. Now that method finally works. AND I've been able to implement 4 more methods - now my linked list is finished! Many thanks for the help.

Lise