Solved

Trying to make sense of $stack[count($stack)] = &$tree;

Posted on 2008-10-20
4
223 Views
Last Modified: 2012-05-05
I'm trying to roll an XML file into a PHP array using Expat Parser, and I'm relatively new to PHP.  I came across the following code: http://us.php.net  /xml_set_element_handler.  (The sample code is about 1/3 the way down the page, author: "rubentrancoso at gmail dot com".)

I'm struggling with the last line of code in the following code block:

$file = "flow/flow.xml";
$depth = 0;
$tree = array();
$tree['name'] = "root";
$stack[count($stack)] = &$tree;

1) As I understand it, count returns the number of elements in the array, but there are no elements in the array, because $stack was just declared.  Right?
2) What's up with &$tree?  

(I have also attached the full sample of the code.)  
<?php
 
$file = "flow/flow.xml";
$depth = 0;
$tree = array();
$tree['name'] = "root";
$stack[count($stack)] = &$tree;
 
function startElement($parser, $name, $attrs) {
   global $depth;
   global $stack;
   global $tree;
  
   $element = array();
   $element['name'] = $name;
   foreach ($attrs as $key => $value) {
        //echo $key."=".$value;
        $element[$key]=$value;
    }
 
   $last = &$stack[count($stack)-1];
   $last[count($last)-1] = &$element;
   $stack[count($stack)] = &$element;
 
   $depth++;
}
 
function endElement($parser, $name) {
   global $depth;
   global $stack;
 
   array_pop($stack);
   $depth--;
}
 
$xml_parser = xml_parser_create();
xml_set_element_handler($xml_parser, "startElement", "endElement");
if (!($fp = fopen($file, "r"))) {
   die("could not open XML input");
}
 
while ($data = fread($fp, 4096)) {
   if (!xml_parse($xml_parser, $data, feof($fp))) {
       die(sprintf("XML error: %s at line %d",
                   xml_error_string(xml_get_error_code($xml_parser)),
                   xml_get_current_line_number($xml_parser)));
   }
}
xml_parser_free($xml_parser);
$tree = $stack[0][0];
echo "<pre>";
print_r($tree);
echo "</pre>";
?>

Open in new window

0
Comment
Question by:jdana
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
  • 2
  • 2
4 Comments
 
LVL 15

Expert Comment

by:MMDeveloper
ID: 22758179
stack[count($stack)] is the same as

$stack[] =

it simply says that the following value will be appended to the end of the array.


&$tree is a reference pointer per say. That means that whatever changes are made to the value in $stack[count($stack)] are replicated automatically to the original $tree value.


If I set this

$a = 5;
$b = &$a;

$b = 0;

now both $b and $a = zero. What happens to $b also happens to $a automatically.
0
 

Author Comment

by:jdana
ID: 22758813
Does $b = &$a; set $b equal to $a BY REFERENCE as opposed to BY VALUE?
0
 
LVL 15

Accepted Solution

by:
MMDeveloper earned 250 total points
ID: 22758924
you are correct.
0
 

Author Closing Comment

by:jdana
ID: 31507826
Thanks!
0

Featured Post

Free Tool: Subnet Calculator

The subnet calculator helps you design networks by taking an IP address and network mask and returning information such as network, broadcast address, and host range.

One of a set of tools we're offering as a way of saying thank you for being a part of the community.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Developers of all skill levels should learn to use current best practices when developing websites. However many developers, new and old, fall into the trap of using deprecated features because this is what so many tutorials and books tell them to u…
3 proven steps to speed up Magento powered sites. The article focus is on optimizing time to first byte (TTFB), full page caching and configuring server for optimal performance.
Explain concepts important to validation of email addresses with regular expressions. Applies to most languages/tools that uses regular expressions. Consider email address RFCs: Look at HTML5 form input element (with type=email) regex pattern: T…
The viewer will learn how to create a basic form using some HTML5 and PHP for later processing. Set up your basic HTML file. Open your form tag and set the method and action attributes.: (CODE) Set up your first few inputs one for the name and …

739 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question