PHP：爲什麼這些輸出不同的結果？

Question

我構建了兩個版本的PHP 7函數，它接受一個數組，並返回顯示原始數組成員的所有排列的數組列表。例如，對於輸入[1,2,3]，期望的輸出將是1,2和3的全部六個排列。

我期望這兩個版本的函數給出相同的輸出，但是不知道爲什麼它們沒有。這是第一個（按預期工作）：

function permutations(array $input): array { 
    $func = function (array $selected, array $chooseFrom, array &$results) 
      use (&$func) { 

    foreach ($chooseFrom as $k => $unchosen): 
     $selectedCopy = $selected; // make a copy 
     $chooseFromCopy = $chooseFrom; // make a copy 

     $selectedCopy[] = $unchosen; // add the next unchosen item to selected list 
     array_splice($chooseFromCopy, $k,1); // remove the item from chooseFrom list 
     $func($selectedCopy, $chooseFromCopy, $results); // recursive call 
    endforeach; 

    // If we've used all items. Add selection to results 
    if (empty($chooseFrom)) $results[] = $selected; 
    }; 

    $results = []; 
    $func([], $input, $results); 
    return $results; 
} 

當我打電話permutations([1,2])我得到預期的結果：[[1,2],[2,1]]。

這是該函數的非工作版本。唯一的區別是在foreach：

function permutations2(array $input): array { 

    $func = function (array $selected, array $chooseFrom, array &$results) 
      use (&$func) { 

    foreach ($chooseFrom as $k => $unchosen):  
     $chooseFromCopy = $chooseFrom; // make a copy 

     $selected[] = $unchosen; // add the next unchosen to the selected list 
     array_splice($chooseFromCopy, $k, 1); // remove the item from chooseFrom list 
     $func($selected, $chooseFromCopy, $results); // recursive call 
    endforeach; 

    // If we've used all items. Add selection to results 
    if (empty($chooseFrom)) $results[] = $selected; 
    }; 

    $results = []; 
    $func([], $input, $results); 
    return $results; 
} 

當我打電話permutations2([1,2])我得到一個壞的結果：[[1,2],[1,2,1]]

爲什麼會出現差異？

Answer 1

問題是關於變量「$ selected」持有for循環的第一次迭代的結果，並且在進入下一次迭代循環之前需要重新初始化。在for循環之前將「$ selected」存儲在另一個變量中（比如說$ tempselected），並在endforeach語句之前用$ tempselected重新初始化「$ selected」變量將使代碼生效。但是這種變化幾乎與函數的工作示例相同:)

<?php 

function permutations2(array $input): array { 

    $func = function (array $selected, array $chooseFrom, array &$results) 
      use (&$func) { 
    $selectedTemp = $selected; 

    foreach ($chooseFrom as $k => $unchosen):  
     $chooseFromCopy = $chooseFrom; // make a copy 

     $selected[] = $unchosen; // add the next unchosen to the selected list 
     array_splice($chooseFromCopy, $k, 1); // remove the item from chooseFrom list 
     $func($selected, $chooseFromCopy, $results); // recursive call 
     $selected = $selectedTemp; 
    endforeach; 

    echo("<br>After For Loop <br>"); 
    // If we've used all items. Add selection to results 
     if (empty($chooseFrom)) { $results[] = $selected; } 
    }; 

    $results = []; 
    $func([], $input, $results); 
    return $results; 
} 
$res = permutations2(['a','b','c']);