How to iterate over multidimensional array using foreach loop?

Introduction

Normally our foreach can sequentially put each key => value in the one-dimensional array Print it out, but if it is a multidimensional array, you need to loop in a nested loop, or implement it recursively, but these methods are not flexible enough, because it is impossible to always Endless nested loops can be solved by recursion, but how to implement it if you just want to use foreach to loop out all the loops?

Implementation method one
Use the iterator class RecursiveIteratorIterator that comes with PHP

  $test_arr = array(1,2,3,array(4,'aa'=>5,6,array(7,'bb'=>8),9,10),11,12); 
  $arrayiter = new RecursiveArrayIterator($test_arr);
  $iteriter = new RecursiveIteratorIterator($arrayiter);
  //直接打印即可按照横向顺序打印出来
  foreach ($iteriter as $key => $val){ 
    echo $key.'=>'.$val; 
  } 
  //结果
  /* 
    0=>1
    1=>2 
    2=>3
    0=>4
    aa=>5
    2=>6
    0=>7
    bb=>8
    4=>9
    5=>10
    4=>11
    5=>12
 */

Implementation method two
Implement one by yourself An iterator class similar to RecursiveIteratorIterator, which implements the horizontal printing function of multi-dimensional arrays

class foreachPrintfArr implements Iterator {
    //当前数组作用域
    private $_items; 
    private $_old_items;
    //保存每次执行数组环境栈
    private $_stack = array(); 

    public function construct($data=array()){
      $this->_items = $data;
    }

    private function _isset(){
      $val = current($this->_items);

      if (empty($this->_stack) && !$val) {
        return false;
      } else {
        return true;
      }  
    }

    public function current() {
      $this->_old_items = null;
      $val = current($this->_items);

      //如果是数组则保存当前执行环境，然后切换到新的数组执行环境
      if (is_array($val)){
        array_push($this->_stack,$this->_items);
        $this->_items = $val;
        return $this->current();
      }

      //判断当前执行完成后是否需要切回上次执行环境
      //(1) 如果存在跳出继续执行
      //(2) 如果不存在且环境栈为空，则表示当前执行到最后一个元素
      //(3) 如果当前数组环境下一个元素不存在,则保存一下当前执行数组环境 $this->_old_items = $this->_items;
      //然后切换上次执行环境 $this->_items = array_pop($this->_stack) 继续循环, 直到当前数组环境下一个
      //元素不为空为止
      while (1) {
        if (next($this->_items)) {  
          prev($this->_items); break;
        } elseif (empty($this->_stack)) {
          end($this->_items); break;
        } else {
          end($this->_items);
          if (!$this->_old_items) 
            $this->_old_items = $this->_items;
          $this->_items = array_pop($this->_stack);
        }
      }

      return $val;
    }

    public function next() {
      next($this->_items);  
    }

    public function key() {
      // 由于 key() 函数执行在 current() 函数之后
      // 所以在 current() 函数切换执行环境 , 会导致切换之前的执行环境最后一个 key
      // 变成切换之后的key , 所以 $this->_old_items 保存一下切换之前的执行环境
      // 防止key打印出错
      return $this->_old_items ? key($this->_old_items) : key($this->_items);
    }

    public function rewind() {
      reset($this->_items);
    }

    public function valid() {                                       
      return $this->_isset();
    }
  }

Internal execution method

1. Foreach loops our custom foreachPrintfArr class, which will automatically call these five internal methods valid (), rewind(), key(), next(), current() We only need to implement these methods.

2. Calling sequence:
1st time => rewind -> valid -> current -> key
2nd~nth time=> next -> valid -> current -> key

   $test_arr = array(1,2,3,array(4,'aa'=>5,6,array(7,'bb'=>8),9,10),11,12);
   $iteriter = new foreachPrintfArr($test_arr);
   foreach ($iteriter as $key => $val){
     echo $key.'=>'.$val;
   } 
   //结果：
   /* 
   0=>1
   1=>2
   2=>3
   0=>4
   aa=>5
   2=>6
   0=>7
   bb=>8
   4=>9
   5=>10
   4=>11
   5=>12
   */

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