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Comparing generators with Iterator objects

The primary advantage of generators is their simplicity. Much less boilerplate code has to be written compared to implementing an Iterator class, and the code is generally much more readable. For example, the following function and class are equivalent:

<?php
function getLinesFromFile($fileName) {
if (!
$fileHandle = fopen($fileName, 'r')) {
return;
}

while (
false !== $line = fgets($fileHandle)) {
yield
$line;
}

fclose($fileHandle);
}

// versus...

class LineIterator implements Iterator {
protected
$fileHandle;

protected
$line;
protected
$i;

public function
__construct($fileName) {
if (!
$this->fileHandle = fopen($fileName, 'r')) {
throw new
RuntimeException('Couldn\'t open file "' . $fileName . '"');
}
}

public function
rewind() {
fseek($this->fileHandle, 0);
$this->line = fgets($this->fileHandle);
$this->i = 0;
}

public function
valid() {
return
false !== $this->line;
}

public function
current() {
return
$this->line;
}

public function
key() {
return
$this->i;
}

public function
next() {
if (
false !== $this->line) {
$this->line = fgets($this->fileHandle);
$this->i++;
}
}

public function
__destruct() {
fclose($this->fileHandle);
}
}
?>

This flexibility does come at a cost, however: generators are forward-only iterators, and cannot be rewound once iteration has started. This also means that the same generator can't be iterated over multiple times: the generator will need to be rebuilt by calling the generator function again.

See Also

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User Contributed Notes 2 notes

up
107
mNOSPAMsenghaa at nospam dot gmail dot com
11 years ago
This hardly seems a fair comparison between the two examples, size-for-size. As noted, generators are forward-only, meaning that it should be compared to an iterator with a dummy rewind function defined. Also, to be fair, since the iterator throws an exception, shouldn't the generator example also throw the same exception? The code comparison would become more like this:

<?php
function getLinesFromFile($fileName) {
if (!
$fileHandle = fopen($fileName, 'r')) {
throw new
RuntimeException('Couldn\'t open file "' . $fileName . '"');
}

while (
false !== $line = fgets($fileHandle)) {
yield
$line;
}

fclose($fileHandle);
}

// versus...

class LineIterator implements Iterator {
protected
$fileHandle;

protected
$line;
protected
$i;

public function
__construct($fileName) {
if (!
$this->fileHandle = fopen($fileName, 'r')) {
throw new
RuntimeException('Couldn\'t open file "' . $fileName . '"');
}
}

public function
rewind() { }

public function
valid() {
return
false !== $this->line;
}

public function
current() {
return
$this->line;
}

public function
key() {
return
$this->i;
}

public function
next() {
if (
false !== $this->line) {
$this->line = fgets($this->fileHandle);
$this->i++;
}
}

public function
__destruct() {
fclose($this->fileHandle);
}
}
?>

The generator is still obviously much shorter, but this seems a more reasonable comparison.
up
23
sergeyzsg at yandex dot ru
10 years ago
I think that this is bad generator example.
If user will not consume all lines then file will not be closed.

<?php
function getLinesFromFile($fileHandle) {
while (
false !== $line = fgets($fileHandle)) {
yield
$line;
}
}

if (
$fileHandle = fopen($fileName, 'r')) {
/*
something with getLinesFromFile
*/
fclose($fileHandle);
}
?>
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