Things to note when using & (and operator) reference assignment in php foreach_PHP Tutorial

foreach can easily modify the elements of the array by adding & before $value, such as:

PHP code

Copy code The code is as follows :

foreach($arr as $value){
$value .= '4';
}

But this usage is also easy to cause It's wrong and not easy to find.

It is more straightforward to look at the example:

PHP code

Copy the code The code is as follows:

$arr = array('a','b','c');
$arr2 = array('d', 'e', ​​'f');

foreach($arr as $value){//Used to use $value or $val
$value .= '4';
}

//After everything is processed, we will add it to the page Template output, first output $arr2
foreach($arr2 as $value){//Used to use $value or $val
//echo $value;
}
//Then output like this $arr;
foreach($arr as $value){//Used to use $value or $val
echo $value, "n";
}
?>

Let’s see if the output result is the same as expected. The result here is:

Copy code The code is as follows:

XML/HTML code
a4
b4
b4

The result is different from what I expected. This is a problem caused by quotation.

At the end of foreach($arr as &$value) array traversal, the reference relationship is not broken. This is equivalent to the reference of the last unit of $value and $arr, which is $arr[2].

Going to foreach($arr2 as $value), the value of $value always changes with the value of the array unit. Since the reference relationship is not broken, the value of $arr[2] also changes accordingly. Until $arr2 is traversed, the value of $value is f, so the value of $arr[2] is also f.
At this time, the value of $arr should be:

XML/HTML code

Copy code The code is as follows:

Array
(
[0] => a4
[1] => b4
[2] => f
)

This is different from the final output we see. Then go to foreach($arr as $value). In the same way, the value of $arr[2] also changes with $value. When the key is 1, that is, $arr[1], $arr[2] The value also becomes the value of $arr[1], which is b4. Then when the key is 2, the value of $arr[2] becomes the value of $arr[2], which is b4. It is the result of output.

So be careful when using references in foreach. You can also disconnect the reference relationship immediately after processing, so that the above situation will not happen later.

PHP code

Copy code The code is as follows:

foreach($arr as $value){
$value .= '4';
}
unset($value);

http://www.bkjia.com/PHPjc/321277.htmlwww.bkjia.comtruehttp: //www.bkjia.com/PHPjc/321277.htmlTechArticleforeach By adding } before $value, but this usage is also easy to cause errors, and it is not easy to find. It is more straightforward to look at the example: PHP code Copy the code The code is as follows: ?php $arr...