The document discusses iterators and how they can be used to traverse collections of data in an object-oriented way. Some key points: 1. An iterator is an object that allows traversal of collection elements through methods like rewind(), next(), current() etc. that abstract the iteration logic. 2. Common iterators discussed include arrays, recursive directories, words in text. Complex iterators can be built by chaining iterators like filters. 3. Iterator patterns avoid explicit looping and make collections agnostic of iteration implementation details. This makes collections reusable and iterable in multiple ways.

1. Uncovering Iterators
SJORS DE VALK
25 NOVEMBER 2010

2. ITERATORS (1)
∂ Black magic?

3. ITERATORS (2)
∂ Holy grail?

4. ITERATORS (3)
“An iterator is an object that allows a programmer
to traverse through all the elements of a
collection.”
∂ Wikipedia

5. ITERATORS (4)
$i = array(1, 2, 3);
reset($i);
while (current($i) !== false) {
echo key($values), current($values);
next($values);
}
∂ Array iteration (old school)

6. ITERATORS (5)
$i = new MyIterator();
$i->rewind();
while ($i->valid()) {
echo $i->key(), $i->current();
$i->next();
}

7. ITERATORS (6)
 rewind()
 valid()
 key()
 current()
 next()
∂ As defined by the Iterator interface

8. ITERATORS (7)
$i = new MyIterator();
foreach ($i as $key => $value) {
echo $key, $value;
}
∂ Methods are called automatically

9. BASICS (1)
$values = array(
‘Cameron Diaz’,
‘Alizée Jacotey’,
‘Britney Spears’,
‘Penélope Cruz’
);

10. BASICS (2)
class NaiveWomenIterator implements Iterator {
function __construct(array $values) {...}
function rewind() {...}
function valid() {...}
function key() {...}
function current() {...}
function next() {...}
}

11. BASICS (3)
class WomenIterator extends ArrayIterator {
// Nothing here
}
∂ Lean and mean

12. BASICS (4)
$i = new WomenIterator($values);
$i = new WomenFilterIterator($i);
foreach ($i as $name) {
echo $name;
}

13. BASICS (5)
class WomenFilterIterator extends FilterIterator {
function accept() {
return strpos($this->current(), ‘z’) !== false;
}
}

14. FIBONACCI (1)
Fibonacci sequence:
0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144...

15. FIBONACCI (2)
$previous = 1;
$current = 0;
while (true) {
echo $current;
$oldCurrent = $current;
$current += $previous;
$previous = $oldCurrent;
}
∂ Classic approach

16. FIBONACCI (3)
$i = new FibonacciIterator();
foreach ($i as $value) {
echo $value;
}
∂ Iterator approach: hides the implementation

17. FIBONACCI (4)
$i = new FibonacciIterator();
$i = new LimitIterator($i, 0, 50);
foreach ($i as $value) {
echo $value;
}
∂ No need to change the original iterator

18. WORDS (1)
$contents = loadFile(‘http://www.gutenberg…’);
$i = new NaiveWordIterator($contents);
foreach ($i as $word) {
echo $word;
}

19. WORDS (2)
$contents = loadFile(‘http://www.gutenberg…’);
$i = new CharacterIterator($contents);
$i = new WordIterator($i);
foreach ($i as $word) {
echo $word;
}

20. WORDS (3)
$contents = loadFile(‘http://www.gutenberg…’);
$i = new CharacterIterator($contents);
$i = new WordIterator($i);
$i = new RegexIterator($i, ‘/god/i’);
foreach ($i as $word) {
echo $word;
}

21. WORDS (4)
$contents = loadFile(‘http://www.gutenberg…’);
$i = new CharacterIterator($contents);
$i = new WordIterator($i);
$i = new RegexIterator($i, ‘/god/i’);
$i = new BigWordsFilterIterator($i, 5);
foreach ($i as $word) {
echo $word;
}

22. WORDS (5)
$contents = loadFile(‘http://www.gutenberg…’);
$i = new CharacterIterator($contents);
$i = new WordIterator($i);
$i = new WordFrequencyIterator($i);
foreach ($i as $word) {
echo $word;
}

23. WORDS (6)
$contents = loadFile(‘http://www.gutenberg…’);
$i = new CharacterIterator($contents);
$i = new WordIterator($i);
$i = new BigWordsFilterIterator($i, 10);
$i = new WordFrequencyIterator($i);
foreach ($i as $word) {
echo $word;
}

24. MP3 (1)
∂ Old school recursive directory iteration
function listFiles($path) {
$files = array();
$handle = opendir($path);
while (false !== ($file = readdir($handle))) {
$files[] = $file;
if (is_dir($path . ‘/’ . $file)) {
$files = array_merge($files, listFiles($path . ‘/’ . $file));
}
}
return $files;
}

25. MP3 (2)
∂ Lean and mean. Returns SplFileInfo
$i = new Mp3RecursiveDirectoryIterator($path);
foreach ($i as $file) {
echo $file->getFilename();
}

26. MP3 (3)
$i = new Mp3RecursiveDirectoryIterator($path);
function render(Iterator $i) {
echo $i->getDepth(), $i->getFilename();
return true;
}
iterator_apply($i, ‘render’, array($i));

27. MP3 (4)
$i = new Mp3RecursiveDirectoryIterator($path);
echo count($i); // Nope
echo $i->count(); // Nope
echo iterator_count($i);

28. MP3 (5)
$i = new Mp3RecursiveDirectoryIterator($path);
$i = new SongsIterator($i);
foreach ($i as $song) {
echo $song->title;
}

29. MP3 (6)
$i = new Mp3RecursiveDirectoryIterator($path);
$i = new SongsIterator($i);
foreach ($i as $song) {
foreach ($song as $property) {
echo $property; // E.g. title, artist
}
}

30. MP3 (7)
∂ No need for a toArray()
class Song implements IteratorAggregate {
function getIterator() {
return new ArrayIterator(
get_object_vars($this)
);
}
}

31. MP3 (8)
$i = new Mp3RecursiveDirectoryIterator($path);
$i = new Mp3ShortSongsFilterIterator($i);
foreach ($i as $file) {
echo $file->getFilename();
}

32. MP3 (9)
$i = new Mp3RecursiveDirectoryIterator($path);
$i = new Mp3ShortSongsFilterIterator($i);
$i = new InfiniteIterator($i);
foreach ($i as $file) {
echo $file->getFilename();
}

33. MOVIES (1)
$i = new ImdbTopMoviesIterator();
$i = new LimitIterator($i, 1, 10);
foreach ($i as $movie) {
echo $movie->rank, $movie->title;
}

34. MOVIES (2)
$i = new ImdbBoxOfficeMoviesIterator($url);
$i = new LimitIterator($i, 1, 10);
foreach ($i as $movie) {
echo $movie->rank, $movie->title;
}

35. MOVIES (3)
$x = new ImdbTopMoviesIterator();
$x = new LimitIterator($x, 1, 10);
$y= new ImdbBoxOfficeMoviesIterator($url);
$y = new LimitIterator($y, 1, 10);
$i = new MultipleIterator();
$i->attachIterator($x);
$i->attachIterator($y);

36. QUESTIONS?

37. ∂ THANKS!