再理解-PHP引用

ermaoL 發(fā)布于2019-06-28 18:44 / 1292人閱讀

摘要：引用本身概念好理解性能也很好但是用好它還是存在著一定的門檻不太好寫。寫本文的起因是這幾天碰到非常好的一個解決方案，讓我重新理解了引用。如果下面的代碼，你看完就能理解了，說明你引用真是學(xué)到家了你也可以直接跳過本文哈。

起因:

日常開發(fā)中，我們會碰到構(gòu)造樹的需求，通過id，pid的關(guān)系去構(gòu)建一個樹結(jié)構(gòu),然后對樹進(jìn)行遍歷等操作。其實(shí)現(xiàn)方式分為兩種: 1. 遞歸， 2. 引用
而這兩個方法的優(yōu)缺點(diǎn)也很明顯。

遞歸實(shí)現(xiàn)起來較容易,但是隨著數(shù)著數(shù)據(jù)量的增大,其性能很低。

引用本身概念好理解,性能也很好,但是用好它還是存在著一定的門檻,不太好寫。

寫本文的起因是,這幾天碰到非常好的一個解決方案，讓我重新理解了引用。通過本文，總結(jié)下自己的學(xué)習(xí)成果.ok,那直接上代碼了。

Practise

如果下面的代碼，你看完就能理解了，說明你引用真是學(xué)到家了, 你也可以直接跳過本文哈~。

function buildTreeByReference($data, $id = "id", $pid = "pid", $child = "children")
    {
        $tmp = []; //以id為健,$value為值的容器，可以很巧妙的判斷根節(jié)點(diǎn)元素
        $tree = [];  
        //利用引用，對$data的數(shù)據(jù)進(jìn)行操作
        foreach ($data as $key => &$value) {
                // 
                $tmp[$value["id"]] = &$value;
                
                if (!isset($tmp[$value["pid"]])) {
                    $tree[] = &$tmp[$value["id"]];
                }else {
                     $temp = &$tmp[$value["pid"]];
                     $temp[$child][] = &$value;
                }
                unset($temp, $value);
        }
        return $tree;
    }

ok，先不說其他的，你先拿下面的數(shù)據(jù)測試下這個方法.

$data= [
            ["id" => 1, "pid" => 0 , "name" => "Universe"],
            ["id" => 2, "pid" => 1 , "name" => "Earth"],
            ["id" => 3, "pid" => 2 , "name" => "China"],
            ["id" => 4, "pid" => 3 , "name" => "Beijing"],
];

補(bǔ)充:這個方法需要注意一點(diǎn)，需要父節(jié)點(diǎn)在前,不適合無序數(shù)據(jù)，所以如果是無序的，先得排序.

如果沒有意外,打印的結(jié)果,應(yīng)該如下:

array(1) {
  [0]=>
  array(4) {
    ["id"]=>
    int(1)
    ["pid"]=>
    int(0)
    ["name"]=>
    string(8) "Universe"
    ["children"]=>
    array(1) {
      [0]=>
      array(4) {
        ["id"]=>
        int(2)
        ["pid"]=>
        int(1)
        ["name"]=>
        string(5) "Earth"
        ["children"]=>
        array(1) {
          [0]=>
          array(4) {
            ["id"]=>
            int(3)
            ["pid"]=>
            int(2)
            ["name"]=>
            string(5) "China"
            ["children"]=>
            array(1) {
              [0]=>
              array(3) {
                ["id"]=>
                int(4)
                ["pid"]=>
                int(3)
                ["name"]=>
                string(7) "Beijing"
              }
            }
          }
        }
      }
    }
  }
}

如果到此，你還想不明白，沒關(guān)系，我們一一來分析下.
其實(shí)要徹底弄明白這個解決方案，需要理解二個部分。

foreach賦值原理

引用的原理

foreach

    $data = ["student", "teacher"];
    foreach ($data as $index => $item) {
    }

注意每次循環(huán)的時候, 是把$data[0]和$data[1] 的“值”復(fù)制一份再賦給 $item

引用(一定要自己動手試驗(yàn)下)

$a = 1;
$b = &$a;
$c = $b;
$c = 2;
猜猜看 $b = ?;

如果引用有疑問,點(diǎn)我

到此，如果你能理解上面foreach和引用，并且能理解這個解決方案的所有執(zhí)行過程，那么恭喜你，你學(xué)的真好! 但如果還是有困難，沒關(guān)系，咱們一步一步踏踏實(shí)實(shí)的來.

Analysis

ok，深吸一口氣，跟著我的思路,咱們一步一步來.

首先咱們看下原函數(shù)

function buildTreeByReference($data, $id = "id", $pid = "pid", $child = "children")
    {
        $tmp = []; #以id為健,$value為值的容器，可以很巧妙的判斷根節(jié)點(diǎn)元素
        $tree = [];  
        #利用引用，對$data的數(shù)據(jù)進(jìn)行操作
        foreach ($data as $key => &$value) {
                
                #&$value取到$data元素對應(yīng)值的引用
                
                $tmp[$value["id"]] = &$value;
                
                #以$value["id"]為鍵，&$value引用為值push到$tmp中,
                #這樣可以巧妙的判斷當(dāng)前元素是否為根節(jié)點(diǎn)
                
                if (!isset($tmp[$value["pid"]])) {
                    #將根節(jié)點(diǎn)push到$tree中
                    $tree[] = &$tmp[$value["id"]];
                    
                }else {
                     #若當(dāng)前元素的父節(jié)點(diǎn)存在于$tmp中, 引用獲取$tmp中對應(yīng)父節(jié)點(diǎn)的值
                     $temp = &$tmp[$value["pid"]];
                     #然后將當(dāng)前元素push到其父節(jié)點(diǎn)的children中
                     $temp[$child][] = &$value;
                }
                #為了不引起變量污染, 引用用完后，需要unset掉
                unset($temp, $value);
        }
        return $tree;
    }

第一次循環(huán)

function buildTreeByReference($data, $id = "id", $pid = "pid", $child = "children")
    {
       # $tmp = [];
       # $tree = [];  
        
       # foreach ($data as $key => &$value) {
                // 
                $tmp[$value["id"]] = &$value;
                
                if (!isset($tmp[$value["pid"]])) {
                    $tree[] = &$tmp[$value["id"]];
                }else {
        #             $temp = &$tmp[$value["pid"]];
        #             $temp[$child][] = &$value;
        #        }
                unset($temp, $value);
        }
        return $tree;
    }

變量情況:
$data[0] = ["id" => 1, "pid" => 0 , "name" => "Universe"];
$tmp[1] = &$data[0];
$tree[] = &$data[0]

第二次循環(huán)

function buildTreeByReference($data, $id = "id", $pid = "pid", $child = "children")
    {
       # $tmp = [];
       # $tree = [];  
        
       # foreach ($data as $key => &$value) {
                // 
                $tmp[$value["id"]] = &$value;
                
        #        if (!isset($tmp[$value["pid"]])) {
        #            $tree[] = &$tmp[$value["id"]];
                }else {
                     $temp = &$tmp[$value["pid"]];
                     $temp[$child][] = &$value;
                }
                unset($temp, $value);
        }
        return $tree;
    }

變量情況:
$data[1] = ["id" => 2, "pid" => 1 , "name" => "Earth"];
$value=&$data[1];
$tmp[2] = &$data[1];
注意：
$temp即&$tmp[1],即和$data[0]指向相同的地址
所以$temp["children"][] = &$value ,操作的結(jié)果是:

$data[
    [  
     "id" => 1,
     "pid" => 0 ,
     "name" => "Universe"
     "children"=>[
                 &$data[1],   //注意:存儲的是引用
         ]
     ]   
     ...
]

4.第三次循環(huán)

function buildTreeByReference($data, $id = "id", $pid = "pid", $child = "children")
    {
       # $tmp = [];
       # $tree = [];  
        
       # foreach ($data as $key => &$value) {
                // 
                $tmp[$value["id"]] = &$value;
                
        #        if (!isset($tmp[$value["pid"]])) {
        #            $tree[] = &$tmp[$value["id"]];
                }else {
                     $temp = &$tmp[$value["pid"]];
                     $temp[$child][] = &$value;
                }
                unset($temp, $value);
        }
        return $tree;
    }

變量情況:
$data[2] = ["id" => 3, "pid" => 2 , "name" => "China"];
$value = &$data[2];
$tmp[3] = &$data[2];
注意：
$temp即&$tmp[2],即和$data[1]指向相同的地址

所以$temp["children"][] = &$value ,操作的結(jié)果是:
這里注意一下：
這是第二次循環(huán)的時候,children中存儲的$data[1]的引用

$data[
    [  
     "id" => 1,
     "pid" => 0 ,
     "name" => "Universe"
     "children"=>[
                 &$data[1],   //注意:存儲的是引用
         ]
     ]   
     ...
]

第三次循環(huán)的的時候，則是$data[1]["children"][] = &$value, 而$value指向的是$data[2]
，所以結(jié)果是:

   $data[
        [  
         "id" => 1,
         "pid" => 0 ,
         "name" => "Universe"
         "children"=>[
                       // &$data[1],   //注意:存儲的是引用
                         [
                           "id" => 2, 
                           "pid" => 1 ,
                           "name" => "Earth"
                           "children" => [
                                   &data[2] //注意:存儲的是引用   
                            ]
                         
                         ] 
                         
                     ]
             ]
         ]   
         ...
    ]

5.第四次循環(huán)

function buildTreeByReference($data, $id = "id", $pid = "pid", $child = "children")
    {
       # $tmp = [];
       # $tree = [];  
        
       # foreach ($data as $key => &$value) {
                // 
                $tmp[$value["id"]] = &$value;
                
        #        if (!isset($tmp[$value["pid"]])) {
        #            $tree[] = &$tmp[$value["id"]];
                }else {
                     $temp = &$tmp[$value["pid"]];
                     $temp[$child][] = &$value;
                }
                unset($temp, $value);
        }
        return $tree;
    }

變量情況:
$data[3] = ["id" => 4, "pid" => 3 , "name" => "Beijing"];
$value = &$data[3];
$tmp[3] = &$data[3];
注意：
$temp即&$tmp[2],即和$data[1]指向相同的地址

所以$temp["children"][] = &$value ,操作的結(jié)果是:
這里注意一下：
這是第三次循環(huán)的時候,children中存儲的$data[2]的引用

   $data[
        [  
         "id" => 1,
         "pid" => 0 ,
         "name" => "Universe"
         "children"=>[
                       // &$data[1],   //注意:存儲的是引用
                         [
                           "id" => 2, 
                           "pid" => 1 ,
                           "name" => "Earth"
                           "children" => [
                                   &data[2] //注意:存儲的是引用   
                            ]
                         
                         ] 
                         
                     ]
             ]
         ]   
         ...
    ]

第四次循環(huán)的的時候，則是$data[2]["children"][] = &$value, 而$value指向的是$data[3]
，所以結(jié)果是:

   $data[
        [  
         "id" => 1,
         "pid" => 0 ,
         "name" => "Universe"
         "children"=>[
                       // &$data[1],   //注意:存儲的是引用
                         [
                           "id" => 2, 
                           "pid" => 1 ,
                           "name" => "Earth"
                           "children" => [
                                  // &data[2] //注意:存儲的是引用 
                                  [
                                      "id" => 3,
                                      "pid" => 2 ,
                                      "name" => "China" 
                                      "children" =>[
                                                 &$data[3];  //注意:存儲的是引用                            
                                      ]
                                  ]  
                            ]
                         
                         ] 
                         
                     ]
             ]
         ]   
         ...
    ]

ok，至此，整個執(zhí)行過程走通了，你懂了嗎？:)

對了，還另外一個方法，也是通過引用的，這個我就不分析，要是理解上面的方法，下面的相對來說簡單些。

    public static function buildTreeByReference1($data, $id = "id", $pid = "pid", $child = "children")
    {
        $tmp = [];
        
        foreach ($data as $key => $value) {
            $tmp[$value[$id]] = $value;
        }
        $tree = [];
        foreach ($tmp as $key => $value) {
                if (isset($tmp[$value["pid"]])) {
                    $tmp[$value["pid"]]["children"][] = &$tmp[$key];
                }else{
                    $tree[] = &$tmp[$key];    
                }
        }
        return $tree;
    }