SplDoublyLinkedList::shift

    (PHP 5 >= 5.3.0, PHP 7, PHP 8)

    SplDoublyLinkedList::shift — Shifts a node from the beginning of the doubly linked list

    Description

    public SplDoublyLinkedList::shift(): mixed

    Parameters

    This function has no parameters.

    Return Values

    The value of the shifted node.

    Errors/Exceptions

    Throws RuntimeException when the data-structure is empty.

    Improve This Page

    Learn How To Improve This Page • Submit a Pull Request • Report a Bug
    ＋add a note

    User Contributed Notes 1 note

    up
    1
    fabs2s at marsatak dot org
    7 years ago
    One may expect SplDoublyLinkedList::shift to properly maintain internal pointers, but this is not the case, this will yield no results, even if you rewind first

    <?php
    while ($splDoublyLinkedList->valid()) {
    yield     $splDoublyLinkedList->shift();
    }
    ?>

    It could be by design, but the following raises some more questions :

    <?php

    $test     = new \SplDoublyLinkedList;
    $dataSet = [
    [    'id' => 1],
    [    'id' => 2],
    [    'id' => 3],
    [    'id' => 4],
    ];

    foreach (    $dataSet as $row) {
    $test->push($row);
    }

    echo     "count: " . $test->count() . PHP_EOL;
    echo     "valid: " . ($test->valid() ? 'true' : 'false') . PHP_EOL;
    echo     "current: " . var_export($test->current(), true) . PHP_EOL;
    echo     "key: " . $test->key() . PHP_EOL;
    echo     "1st shift: " . var_export($test->shift(), true) . PHP_EOL;
    echo     "count: " . $test->count() . PHP_EOL;
    echo     "valid: " . ($test->valid() ? 'true' : 'false') . PHP_EOL;
    echo     "current: " . var_export($test->current(), true) . PHP_EOL;
    echo     "key: " . $test->key() . PHP_EOL;
    echo     "2nd shift: " . var_export($test->shift(), true) . PHP_EOL;
    echo     "count: " . $test->count() . PHP_EOL;
    echo     "valid: " . ($test->valid() ? 'true' : 'false') . PHP_EOL;
    echo     "current: " . var_export($test->current(), true) . PHP_EOL;
    echo     "key: " . $test->key() . PHP_EOL;
    echo     "rewinding... " . PHP_EOL;
    $test->rewind();
    echo     "current: " . var_export($test->current(), true) . PHP_EOL;
    echo     "2nd shift: " . var_export($test->shift(), true) . PHP_EOL;
    echo     "count: " . $test->count() . PHP_EOL;
    echo     "valid: " . ($test->valid() ? 'true' : 'false') . PHP_EOL;
    echo     "current: " . var_export($test->current(), true) . PHP_EOL;
    echo     "key: " . $test->key() . PHP_EOL;
    ?>

    will result in :
    <?php

    /*
    count: 4
    valid: false <== First thing to note, you have no valid pointer unless you rewind first
    current: NULL <== hence no current
    key: 0 <== but we have a valid key
    offsetGet(key): array ( <== indeed
    'id' => 1,
    )
    1st shift: array ( <== and shift does return first row
    'id' => 1,
    )
    count: 3 <== and count is maintained as expected
    valid: false <== but internal pointer was left outside valid range
    current: NULL <== hence no current again
    key: 0 <== but we still have a valid key
    offsetGet(key): array ( <== indeed
    'id' => 2,
    )
    2nd shift: array ( <== and shift does return first row
    'id' => 2,
    )
    count: 2 <== and count is maintained as expected
    valid: false <== still not valid etc ..
    current: NULL
    key: 0
    offsetGet(key): array (
    'id' => 3,
    )
    rewinding... <== now rewind
    current: array ( <== yay a current
    'id' => 3,
    )
    3rd shift: array ( <== shift ok
    'id' => 3,
    )
    count: 1 <== count ok
    valid: true <== ouch valid
    current: NULL <== with no current
    key: 0 <== still our key is valid :o
    offsetGet(key): array (
    'id' => 4,
    )
    */
    ?>

    Conclusion : I may be missing something about why SplDoublyLinkedList::shift is not maintaining proper internal index in the first place, but I find it even more confusing to be able to end up with a valid valid() and a valid key() and no current() while there is obviously one.

    Tested on php 5.6.30 & 7.1.2 with the exact same result.
    ＋add a note
    To Top