Summary of foreach() usage in PHP_PHP tutorial

A summary of the usage of foreach() in PHP

This article mainly introduces the usage of foreach() in PHP and related examples in detail. It is very detailed and can help you if necessary. Partners can refer to it.

PHP 4 introduced the foreach structure, which is very similar to Perl and other languages. This is just a convenient way to iterate over 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.

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　5foreach (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 the $val variable. This method can only obtain The value in the array, but not the subscript index value of the array. For example:

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　4$myArray=array("1"=>"val1","2"=>"val2","3"=>"val3");

foreach($myArray as $val) {

print($val." ");

　}

The result will be output: val1 val2 val3

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

Look at positive examples:

First look at a simple one-dimensional array:

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4$myArray=array("1"=>"val1","2"="val2","3"=>"val3");

foreach($myArray as $key=>$val) {

print($key."=>".$val.";");

　}

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

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　17$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("

");

foreach($myArray as $key=>$val) {

print("

• ".$key."
");

　if (is_array($val)) { //Determine whether the value of $val is an array, if so, enter the lower level traversal

print("

");

foreach($val as $key=>$val) {

print("

• ".$key."=>".$val."
");

　}

print("

");

　}

　}

print("

");

Output result:

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　121

　11=>val11

　12=>val12

　13=>val13

　2

　21=>val21

　22=>val22

　23=>val23

　3

　31=>val31

　32=>val32

　33=>val33

　

And
• is a tag, which is used to display 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 in the traversal to facilitate the second-level array traversal.

  Another example to help solve the puzzle

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  $a = array("1"=>"Chinese","2"=>"Mathematics","3"=>"English");

  　$b = array("1"=>"95","2"=>"99","3"=>"92");

  foreach($a as $key=>$value){

  echo $value;

  echo $b[$key]."
  ";

  　}

  　?>

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

  Why is this?

  $a = array("1"=>"Chinese","2"=>"Mathematics","3"=>"English");

  The one above and the one below are 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];

  Right?

  If there is no doubt, let’s continue!!!

  -----------------------------

  Simply say foreach

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

  The subscript is the $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

  Remember, no matter how it changes, the output method of the array will always be $a[1], not $a['Chinese']

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

  There are two ways to use foreach():

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  　31: 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, which is what you get back in the next loop. The next element.

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  　32: foreach(array_name as $key => $value){

  statement;

  　}

  The difference between this and the first method is that there is an additional $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 to the variable $key in each loop. . The key value can be a subscript value or a string. For example, "0" in book[0]=1, "id" in book[id]="001".

  The above is the entire content of this article, I hope you all like it.

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