循环遍历数组并比较时间戳

Question

I have two arrays that I like to loop through and compare the dates between each array.

$newArray = Array (
    [0] => Array (
            [id] => 1
            [date] => 26-11-2013 9:30:56 PM
        )
    [1] => Array (
            [id] => 2
            [date] => 30-11-2013 11:20:12 AM
        )
    [2] => Array (
            [id] => 3
            [date] => 26-11-2013 9:30:56 PM
        )
    [3] => Array (
            [id] => 4
            [date] => 30-11-2013 11:20:12 AM
        )
}

$oldArray = Array (
    [0] => Array (
            [id] => 1
            [date] => 26-11-2013 9:30:56 PM
        )
    [1] => Array (
            [id] => 2
            [date] => 26-11-2013 9:30:56 PM
        )
    [2] => Array (
            [id] => 3
            [date] => 26-11-2013 9:30:56 PM
        )
}

foreach ($newArray as $newPhoto) {
    foreach ($oldArray as $oldPhoto) {
        if (strtotime($newPhoto['date']) != strtotime($oldPhoto['date'])) {
            // download new photo
        }
    }
}

I realize that placing the foreach with a foreach is not going to cut it. What is the best way to loop through each of these arrays and compare the dates?

The $newArray has the latest photos and compares with the $oldArray if the timestamps do not match or there is a new one in the list, download the new images.

In the example, I would be downloading the second and fourth images and ignoring the others.

Answer 1

I'd do it like this. It does not make any assumptions about the indices of either array, and will be still be fairly efficient as $oldArray grows in size.

// index $oldArray by id for speed
$index = array();
foreach ($oldArray as $photo) {
    $index[$photo['id']] = $photo['date'];
}

// iterate through new photos, checking if each one needs downloading
foreach ($newArray as $photo) {
    if (!isset($index[$photo['id']]) // photo is not in $oldArray
            || strtotime($photo['date']) > strtotime($index[$photo['id']])) { // photo is in $oldArray, but a new version is available
        // download new photo
    }
}