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php数组指针用法详解

Jun 08, 2016 pm 05:25 PM
arr current dump nbsp var

<script>ec(2);</script>

先给出一条结论,然后我们在用代码来证明这个结论吧。$arrtmp=$arr;在这个赋值表达式中我把$arr叫做赋值数组,把$arrtmp叫做被赋值数组。数 组在赋值时,如果赋值数组的数组指针已经指向了数组末尾,则赋值之后赋值数组的数组指针会被重置,指向数组第一个元素;如果在赋值时,赋值数组的数组指针 没有指向数组末尾,而是指向了任何一个有效的数组元素,那么在赋值之后赋值数组的数组指针是不会被重置的,而是保留其原来指向的元素。在赋值之后,被赋值 数组不仅有了赋值数组的值,而且赋值数组的数组指针指向了那个元素,被赋值的数组也会指向自己中值相同的那个元素。
demo1:

$arr = array('var1'=>1,'var2'=>2,'var3'=>3,'var4'=>4,'var5'=>5);
while( list($key,$value) = each($arr) )
{
    if($value == 4) break;
}
var_dump(current($arr));

$arr1 = $arr;

var_dump(current($arr));
var_dump(current($arr1));
?>


demo1 的执行结果是:int(5) int(5) int(5) 。从这个结果可以看出,赋值前后$arr的数组指针位置没有发生任何变化,$arr1不仅值跟$arr相同,而且数组指针所指向的元素值也是相同的。现在 用上述结论来解释这个结果,在while循环中,有一个if判断语句,目的是不让$arr的数组指针指向数组末尾,而是保留在一个有效的位置。 在$value=4时会跳出循环,而each这个函数会将数组指针向前移动一位,这就导致了$arr的数组指针指向了第5个元素,所以在赋值之 前,current($arr)的结果是5,赋值之后,由于在赋值之前$arr的当前指针并没有指向末尾,因此在赋值之后不会将$arr的数组指针进行重 置,而是保留了其原有的位置,因此在赋值之后使用current($arr)的结果仍然是5。赋值时$arr1不仅获得了$arr的值,而且数组指针指向 的元素和$arr的相同,二者都是5。
demo2:

$arr = array('var1'=>1,'var2'=>2,'var3'=>3,'var4'=>4,'var5'=>5);
while( list($key,$value) = each($arr) )
{
    //if($value == 4) break;
}
var_dump(current($arr));

$arr1 = $arr;

var_dump(current($arr));
var_dump(current($arr1));
?>


demo2中我们将 if($value == 4) break; 这一句注释掉了,目的很简单,就是通过each将$arr的数组指针位置指向数组末尾。
demo2 的执行结果:bool(false) int(1) bool(false) 。如果数组指针对应的元素为0,“”,或者不是一个有效的值时,current函数会返回false,$arr的值中没有为0或者“”的情况,因此可以断 定是因为数组指针指向了一个无效的元素而导致current返回了一个false。换句话说就是可以确定在while循环完成之后,$arr的数组指针已 经指向了数组的末尾。所以我们看到在赋值之前current($arr)的值是false,而赋值之后current($arr)的值变成了1,说明赋值 之后$arr的数组指针被重置了,指向了数组的第一个元素。current($arr1)的值为false,说明赋值之后$arr1让然保留了赋值之 前$arr的数组指针指向的元素。

不知道描述清楚没有,还是将工作中问题简化一下,用下面代码说明一下吧。

          $arr = array('var1'=>1,'var2'=>2,'var3'=>3,'var4'=>4,'var5'=>5);
          while( list($key,$value) = each($arr) )
          {
               $$key = $value;
         }
         //下面这一步操作很关键,问题就出在这里,遍历完之后,对数组添加一个值。
         $arr['var6'] = 6;
        
        func($arr);
       
       function func($arrtmp)
       {
              while(list($key,$value) = each($arrtmp) )
              {
                     $$key = $value;
              }
              var_dump($var1);
              var_dump($var2);
              var_dump($var3);
              var_dump($var4);
              var_dump($var5);
              var_dump($var6);
       }
?>


输出结果:NULL NULL NULL NULL NULL int(6).

 

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