PHP object access issue

Question

I wrote this implementation of stack and partial implementation of queue using two stacks. Almost everything works as expected besides one thing. For some issue when I var_dump the result of dequeue it returns NULL when when I var_dump the result inside of dequeue it returns a boolean value as expected. Can anyone explain where is the difference coming from?

<?php


class stack
{
    private $stack = array();
    function push($value)
    {
        $this->stack[] = $value;
    }

    function pop()
    {
        if ($this->isEmpty())
            throw new RunTimeException("Stack is empty");   
        $top = $this->stack[count($this->stack)-1];
        unset($this->stack[count($this->stack)-1]);
        $this->stack = array_values($this->stack);
        return $top;
    }
    function isEmpty()
    {
        return empty($this->stack) ? true : false;
    }
    function peak()
    {   
        $top = $this->stack[count($this->stack)];
        return $top;
    }
    function printr()
    {
        print_r($this->stack);
    }
}



class queue
{
    function __construct()
    {
        $this->stack1 = new Stack();
        $this->stack2 = new Stack();
    }

    function push($value)
    {
        $this->stack1->push($value);
    }       

    function dequeue()
    {
        if (!$this->stack2->isEmpty())
        {
            $this->stack2->printr();
            $pop = $this->stack2->pop();
            var_dump($pop);
            return $pop;
        }   
        else if (!$this->stack1->isEmpty())
        {
            do 
            {
                $pop = $this->stack1->pop();
                $this->stack2->push($pop);
            }   
            while ($this->stack1->isEmpty() === false);
            $this->dequeue();
        }
        else
        {
            throw new RunTimeException("Queue is empty");   
        }

    }
    function isEmpty()
    {
        if (($this->stack1->isEmpty()) AND ($this->stack2->isEmpty()))
        {
            return true;
        }
        else
        {
            return false;
        }
    }
    function peak()
    {

    }
}


$myQueue = new queue();
$myQueue->push(1);
$myQueue->push(2);
var_dump($myQueue->dequeue());

` ? true : false` - why are you doing this? empty already returns a boolean — ThiefMaster
– ThiefMaster, Commented Aug 30, 2015 at 8:11

Anid Monsur · Accepted Answer · 2015-08-30 03:04:56Z

1

Since you're using recursion, you need to return that value in your second if block.

do { 
   ...
}
while ($this->stack1->isEmpty() === false);
return $this->dequeue();

answered Aug 30, 2015 at 3:04

Anid Monsur

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Anthony Rutledge · Accepted Answer · 2015-08-30 08:05:50Z

1

do 
{
    $pop = $this->stack1->pop();
    $this->stack2->push($pop);
}   
while ($this->stack1->isEmpty() === false);
$this->dequeue();

The client/calling code will not see the result of $this->dequeue() (above) because, due to recursion, you will be (at least) two levels down (removed) from the client/calling code. This is why you are getting a null result in the client/calling code.

edited Aug 30, 2015 at 8:05

answered Aug 30, 2015 at 3:39

Anthony Rutledge

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