通过继续值对Usort排序多维数组

Question

I have an array:

$cards = [
    [
        "from" => "Barcelona",
        "to" => "Gerona Airport",        
    ],
    [
        "from" => "Stockholm",
        "to" => "New York JFK",          
    ],
    [
        "from" => "Gerona Airport",
        "to" => "Stockholm",          
    ],
    [
        "from" => "Madrid",
        "to" => "Barcelona",          
    ]
];

I need to sort it to make a continuing trip, where the from matches the previous to , or be set as the start if not, like:

Array (
    [0] => Array ( [from] => Madrid [to] => Barcelona )  
    [1] => Array ( [from] => Barcelona [to] => Gerona Airport )
    [2] => Array ( [from] => Gerona Airport [to] => Stockholm ) 
    [3] => Array ( [from] => Stockholm [to] => New York JFK ) 
)

I am trying to use usort :

usort($cards, function ($a, $b) {
          return ( $a["to"] === $b["from"] ) ? -1 : 1;
      });

print_r($cards);

But it's does not sort in the manner described. Any ideas?

Answer 1

Hopefully a better solution will come along, but if you sort as many times as there are elements in the array and return equal or greater then it works; even if you don't know the beginning or if the beginning is not the first element. Because of the way usort traverses and compares, returning equal or less does not work:

foreach($cards as $card) {
    usort($cards, function ($a, $b) {
                      return ( $a["to"] === $b["from"] ) ? 0 : 1;
                  });
}

It may work with fewer iterations, but comparing each against all others seems safe.

Answer 2

You might take a recursive approach and specify the city to start from.

By default, you start at Madrid with doSort($cards, $result); .

If you want to get the continuing trip from "Gerona Airport", you could start it with doSort($cards, $result, "Gerona Airport");

$cards = [
    [
        "from" => "Barcelona",
        "to" => "Gerona Airport",
    ],
    [
        "from" => "Stockholm",
        "to" => "New York JFK",
    ],
    [
        "from" => "Gerona Airport",
        "to" => "Stockholm",
    ],
    [
        "from" => "Madrid",
        "to" => "Barcelona",
    ]
];
$result = [];

function doSort($cards, &$result, $start = "Madrid") {
    foreach ($cards as $key => $card) {
        if ($card["from"] === $start) {
            $result[] = $card;
            doSort($cards, $result, $card["to"]);
        }
    }
}
doSort($cards, $result);
print_r($result);

Php output demo

That will give you:

Array
(
    [0] => Array
        (
            [from] => Madrid
            [to] => Barcelona
        )

    [1] => Array
        (
            [from] => Barcelona
            [to] => Gerona Airport
        )

    [2] => Array
        (
            [from] => Gerona Airport
            [to] => Stockholm
        )

    [3] => Array
        (
            [from] => Stockholm
            [to] => New York JFK
        )

)

Answer 3

here is a way. Finding the start, and search the path, by iteration :

// 1. Find the start
$start = null;
$starts = array_column($cards,'from');
$ends = array_column($cards,'to');
foreach ($cards as $card) {
    if (!in_array($card['from'], $ends)) $start=$card;
}
// 2. Find way
$way=[$start];
while (count($cards)>1) {
    $found = false ;
    foreach ($cards as $k => $card) {
        if ($card['from'] == $start['to']) {
            $way[] = $card;
            $start = $card;
            unset($cards[$k]);
            $found = true ;
            break 1;
        }
    }
    if (!$found) { echo "Impossible to use all cards.\n" ; break ; }
}
print_r($way);

Outputs:

    Array (
    [0] => Array ([from] => Madrid [to] => Barcelona )
    [1] => Array ([from] => Barcelona [to] => Gerona Airport )
    [2] => Array ([from] => Gerona Airport [to] => Stockholm )
    [3] => Array ([from] => Stockholm [to] => New York JFK )
)