Summary of usage and extension examples of php foreach()

PHP 4 introduced the foreach construct, much like Perl and other languages. This is just a convenient way of traversing an array. foreach can only be used with arrays, and an error will occur when trying to use it with other data types or an uninitialized variable. There are two syntaxes, the second being a less important but useful extension of the first.

foreach (array_expression as $value)
  statement

foreach (array_expression as $key => $value)
  statement

The first format iterates over the given array_expression array. Each time through the loop, the value of the current cell is assigned to $value and the pointer inside the array is moved forward one step (so the next cell will be obtained in the next loop).

The second format does the same thing, except that the key name of the current unit will also be assigned to the variable $key in each loop.

Let’s look at the first statement first. This statement is relatively simple. array_expression refers to an array. Expression. The as $val statement will sequentially obtain the value of the array and save it to $val. In variables, this method can only obtain the value in the array, but not the subscript index value of the array. For example:

$myArray=array("1"=>"val1","2"=>"val2","3"=>"val3");
foreach($myArray as $val) {
   print($val." ");
}

The result will be output: val1 val2 val3

Let’s look at the second format. In addition to getting the value of the elements in the array like the first format, the second format can , you can also get the index value of the element and save it to the $key variable. If the index value of the array has not been manually set, it will return to the system default setting value.
See the positive example:

Let’s first look at a simple one-dimensional array:

$myArray=array("1"=>"val1","2"="val2","3"=>"val3");
foreach($myArray as $key=>$val) {
   print($key."=>".$val.";");
}

The program will output: 1=>val1;2=>val2;3=>val3;, next we Let’s look at a more complicated two-dimensional array traversal. The program is as follows:

$myArray=array(
   "1"=>array("11"=>"val11","12"=>"val12","13"=>"val13"),
   "2"=>array("21"=>"val21","22"=>"val22","23"=>"val23"),
   "3"=>array("31"=>"val31","32"=>"val32","33"=>"val33")
);
print("<ul>");
foreach($myArray as $key=>$val) {
   print("<li>".$key."</li>");
   if (is_array($val)) {   //判断$val的值是否是一个数组，如果是，则进入下层遍历
     print("<ul>");
    foreach($val as $key=>$val) {
       print("<li>".$key."=>".$val."</li>");
     }
     print("</ul>");
   }
}
print("</ul>");

Output result:

  1
    11=>val11
    12=>val12
    13=>val13
  2
    21=>val21
    22=>val22
    23=>val23
  3
    31=>val31
    32=>val32
    33=>val33

and
• are labels, their functions It shows a solid dot and a hollow dot.
  Since the above is a two-dimensional array, the $val value obtained after the first traversal will be an array, so I added a judgment to the traversal for second-level array traversal.

  Another example to solve the puzzle

  <?php
  $a = array("1"=>"语文","2"=>"数学","3"=>"英语");
  $b = array("1"=>"95","2"=>"99","3"=>"92");
  foreach($a as $key=>$value){
    echo $value;
  echo $b[$key]."<br>";
  }
  ?>

  The question is why does the value in the output array $b use $b[$key] instead of $b[$value]?

  Why is this?

  $a = array("1"=>"Chinese","2"=>"Mathematics","3"=>"English");
  The one above and the one below It’s exactly the same

  $a[1]="Chinese";
  $a[2]="Mathematics"
  $a[3]="English"

  How do we output the above array?

  It must be echo $a[1];

  －－－－－－－－－－－－－－－－－－－－－－－－－－－－ -

  Simply put foreach

  Its format is like this foreach (array name as subscript => value)

  The subscript is In $a[1] above, the 1 here is the subscript of the array!

  At this point you should understand why $a[$key] is output like this

  You remember that no matter how you change it, the output method of the array will always be $a[1], it won’t It's $a['language']

  ====================================== ============================

  foreach() has two uses:

  1： foreach(array_name as $value){
      statement;
    }

  The array_name here is the name of the array you want to traverse. In each loop, the value of the current element of the array_name array is assigned to $value, and the subscript inside the array moves down one step, that is, the next loop Return the next element.

  2：foreach(array_name as $key => $value){    
      statement;  
   }

  The difference between this and the first method is that there is an extra $key, that is, in addition to assigning the value of the current element to $value, the key value of the current element will also be assigned in each loop. Give the variable $key. The key value can be a subscript value or a string. For example, "0" in book[0]=1, "id" in book[id]="001".

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