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.

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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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