在 PHP 中翻转 two-dimensional 关联数组

Question

我需要将 parents 到 children 的数组转换为 children 到 parents 的数组。例如，我有一个这样的数组：

[
    1 => [a,b,c],
    2 => [b,c,d],
    3 => [c,d,e],
]

我想把它变成这样：

[
    a => [1],
    b => [1,2],
    c => [1,2,3],
    d => [2,3],
    e => [3]

]

有没有不使用嵌套 foreach 循环来完成这个任务的方法？如果没有，最有效的方法是什么？

提前致谢！

Answer 1

使用 array_merge_recursive、array_combine 和 array_fill 函数的简短解决方案：

$arr = [
    1 => ['a','b','c'],
    2 => ['b','c','d'],
    3 => ['c','d','e'],
];

$result = [];
foreach ($arr as $k => $v) {
    $result = array_merge_recursive($result, array_combine($v, array_fill(0, count($v), [$k])));
}

print_r($result);

输出：

Array
(
    [a] => Array
        (
            [0] => 1
        )

    [b] => Array
        (
            [0] => 1
            [1] => 2
        )

    [c] => Array
        (
            [0] => 1
            [1] => 2
            [2] => 3
        )

    [d] => Array
        (
            [0] => 2
            [1] => 3
        )

    [e] => Array
        (
            [0] => 3
        )
)

Answer 2

就 "efficiency" 而言，我认为在这种情况下，嵌套循环更好：

$arr = [1 => ['a','b','c'],
        2 => ['b','c','d'],
        3 => ['c','d','e']];

$result = [];
foreach ($arr as $key => $value) {
    foreach ($value as $v) {
        $result[$v][] = $key;
    }
}

var_dump($result);

正在尝试通过 array_map may turn out to be slower, at least according to this answer 等其他功能发挥创意。可能值得运行您自己的一些基准。

Answer 3

使用闭包和 array_map（只能希望 array_map 比等效的 for 循环执行得更快......它不应该是一个本地函数吗？）。

$multimap=[
    1 => [a,b,c],
    2 => [b,c,d],
    3 => [c,d,e],
];

$result=[];
foreach($multimap as $k=>$arr) {
  $callme=function($e) use (&$result, $k) {
    if( ! array_key_exists ($e, $result) ) {
      $result[$e]=[];
    }
    $result[$e][]=$k;
    return $e; // not that it matters what is returned, we're after the side-effects
  };
  array_map($callme, $arr);
}

// just as yet another alternative to var_dump/print_r
echo json_encode($result /*, JSON_PRETTY_PRINT */)."\n";

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