One of my function calls returned a collection of pairs of integers and I was wondering whether to store the pair as an array of two named values (as in array('value1' => $value1, 'value2' => $value2)) or a PHP5 class (as in class ValuePair { var $value1; var $value2; }). As the number of pairs could be quite large, I thought I'll optimize for memory. Based on experience with compiled languages such as C/C++ and Java, I expected the class based implementation to take less space. Based on a simple memory measurement program, as I'll explain later, this expectation turned out to be misplaced. Apparently PHP implements both arrays and objects as hash tables and in fact, objects require a little more memory than arrays with same members. In hindsight, this doesn't appear so surprising. Compiled languages can convert member accesses to fixed offsets but this is not possible for dynamic languages.

But what did surprise me was the amount of space being used for an array of two elements. Each array having two integers, when placed in another array representing the collection, was using around 300 bytes. The corresponding number for objects is around 350 bytes. I did some googling and found out that a single integer value stored within an PHP array uses 68 bytes: 16 bytes for value structure (zval), 36 bytes for hash bucket, and 2*8 = 16 bytes for memory allocation headers. No wonder an array with two named integer values takes up around 300 bytes.

I am not really complaining -- PHP is not designed for writing data intensive programs. After all, how much data are you going to display on a single web page. But it is still nice to know the actual memory usage of variables within your program. What if your PHP program is not generating an HTML page to be rendered in the browser but a PDF or Excel report to be saved on disk? Would you want your program to exceed memory limit on a slightly larger data set?

Coming back to the original problem -- how should I store a collection pair of values? array of arrays or array of objects? For memory optimization, the answer may be to have two arrays, one for each value.

For those who care for nitty-gritties, here is the program I used for measurements:

<?php
class EmptyObject { };
class NonEmptyObject {
  var $int1;
  var $int2;
  function NonEmptyObject($a1, $a2){
    $this->int1= $a1;
    $this->int2= $a2;
  }
};
$num = 1000;
$u1 = memory_get_usage();
$int_array = array();
for ($i = 0; $i < $num; $i++){
  $int_array[$i] = $i;
}
$u2 = memory_get_usage();
$str_array = array();
for ($i = 0; $i < $num; $i++){
  $str_array[$i] = "$i";
}
$u3 = memory_get_usage();
$arr_array = array();
for ($i = 0; $i < $num; $i++){
  $arr_array[$i] = array();
}
$u4 = memory_get_usage();
$obj_array = array();
for ($i = 0; $i < $num; $i++){
  $obj_array[$i] = new EmptyObject();
}
$u5 = memory_get_usage();
$arr2_array = array();
for ($i = 0; $i < $num; $i++){
  $arr2_array[$i] = array('int1' => $i, 'int2' => $i + $i);
}
$u6 = memory_get_usage();
$obj2_array = array();
for ($i = 0; $i < $num; $i++){
  $obj2_array[$i] = new NonEmptyObject($i, $i + $i);
}
$u7 = memory_get_usage();

echo "Space Used by int_array: " . ($u2 - $u1) . "\n";
echo "Space Used by str_array: " . ($u3 - $u2) . "\n";
echo "Space Used by arr_array: " . ($u4 - $u3) . "\n";
echo "Space Used by obj_array: " . ($u5 - $u4) . "\n";
echo "Space Used by arr2_array: " . ($u6 - $u5) . "\n";
echo "Space Used by obj2_array: " . ($u7 - $u6) . "\n";
?>

[pankaj@fc7-dev ~]$ php -v
PHP 5.2.4 (cli) (built: Sep 18 2007 08:50:58)
Copyright (c) 1997-2007 The PHP Group
Zend Engine v2.2.0, Copyright (c) 1998-2007 Zend Technologies
[pankaj@fc7-dev ~]$ php -C memtest.php
Space Used by int_array: 72492
Space Used by str_array: 88264
Space Used by arr_array: 160292
Space Used by obj_array: 180316
Space Used by arr2_array: 304344
Space Used by obj2_array: 349144
[pankaj@fc7-dev ~]$

So why did I choose this particular title? No, I didn't intend to write everything I know about Ajax. It is just a link-bait. Seems to have worked quite well for others. Might work for me as well.

What I really want to do in this post is to write a short review of "Ajax -- The Definitive Guide", a book published by O'Reilly. Those who are familiar with Oreilly's The Definitive Guide series know that these books have a reputation of being very comprehensive and all encompassing about the chosen topic. This certainly seems to be the case for a number of books in this series on my bookshelf, such as "JavaScript: The Definitive Guide" and "SSH, The Secure Shell: The Definitive Guide". But a definitive guide on something like Ajax? It would have to cover a lot of stuff, in all their fullness and fine details, to do justice to the title: the basics of Ajax interactions, (X)HTML, JavaScript, XML, XmlHttpRequest, CSS, DOM, browser idiosyncrasies, Ajax programming style and design patterns, tips-n-tricks, numerous browser side Ajax libraries such as prototype, YUI library, jQuery etc. and their integration with server side frameworks such as RoR, Drupal etc. The list is fairly long, if not endless. And each topic worthy of a book by itself.

On the other hand, the book does provide good introduction to basic concepts, is quite readable, includes a lot of source code for non-trivial working programs and lists relevant resources, such as Ajax libraries, frameworks and applications, in its References section. I especially liked the "chat" and "whiteboard" application that allows two or more users to share a whiteboard and chat through their browsers.

Okay, so how does this book compares with other books on the same topic? This is a tough question, for I haven't been paying attention to most books that have come out on this topic. Though there is a answer, and it comes from this Amazon Sales Rank comparison chart:

A higher Sales Rank for an item implies that more people are buying it from Amazon. This doesn't tell how well a particular book will meet your needs but just that the high ranking items, in general, are being bought by more people than the low ranking ones. The above chart does indicate that Ajax -- The Definitive Guide is outselling its rivals, at least at the time of this review (March 17-18, 2008).]]> tag:pankaj-k.net,2007:/weblog//1.188 2007-11-27T22:24:21Z 2007-11-27T23:06:30Z

preg_match('/Name: (.+), Age: (\d+)/', $text, $matches);

preg_match('/Name: (?P<Name>.+), Age: (?P<Age>\d+)/', $text, $matches);

a.b
aab
aaa

$ sort list.txt

Expected           sort output
=======           ========
a.b                   aaa
aaa                   aab
aab                   a.b

php > $a = array("a.b", "aab", "aaa");
php > sort($a);
php > print_r($a);
Array
(
    [0] => a.b
    [1] => aaa
    [2] => aab
)

function addEvent( obj, type, fn ) { 
  if ( obj.attachEvent ) { 
    obj['e'+type+fn] = fn; 
    obj[type+fn] = function()
      {obj['e'+type+fn]( window.event );} 
    obj.attachEvent( 'on'+type, obj[type+fn] ); 
  } else 
    obj.addEventListener( type, fn, false ); 
}

function create_handler(func, arg1){
  return function(event){ 
    return lfunc.call(null, 
      event || window.event, arg1); 
  }
}

function long_function(event, arg1){
 ... tens of lines of code ...
}
addEvent(obj, 'mousemove', 
  create_handler(long_function, arg1));

I use a compaq nc4000 laptop running Windows XP as the main system and have a number of other machines running either WIndows or Linux. The main Windows XP system also includes Cygwin, a collection of GNU tools such as ssh, svn, cvs, tar, wget etc. Use of ssh keys allows ssh sessions to the Linux system without the need to enter a password. All my data files, including Outlook PST files, are under a single directory in C drive.

Copying a directory tree from Windows XP system to a Linux host machine is fairly straight-forward with rsync (available within Cygwin):

rsync -avz <datadir> <user>@<host>:backup

Turning this simple command into a daily backup is fairly straight forward: create a command file backup.cmd with following text (or adapt to your setup):

c:
cd \
rsync -avz <datadir> <user>@<host>:backup >> backup.log

That is all you need to do. Just look at c:\backup.log once in a while to make sure that the new or modified files are indeed being transfered.

No backup process is complete without the restore capability. So, here is the restore equivalent:

rsync -avz  <user>@<host>:backup/<datadir> .