PHP: Detailed explanation of foreach and in_array usage examples

php is very efficient in development, which is understandable, but it is sacrificing execution efficiency. php arrayThe function is very powerful, but you should also consider it more and try several situations just in case. Here, I will briefly talk about the two pitfalls encountered. If you find more, please add them!

1. foreach provides a simple way to traverse an array, which can easily read the contents of data or objects, but the official documentation says , since foreach relies on an internal array pointer, modifying its value in a loop may cause unexpected behavior. So, basically,
1. Don't try to modify the value inside the loop, otherwise the result will be beyond what you want;
2. Using '&' is a safe way, although it is rarely used. However, when used, after the reference ends, the unset function should be called immediately to destroy the variable. Otherwise, in the following code, if this variable is used again, the last value of the loop will be modified, thus Unexpected values ​​were obtained. For example: when the list is looped out, there will be many messy values ​​or null values ​​in the last line of output. Using unset solves this problem.

Let’s look at an example

<?php 
/*-------------------------------------------------------------------------*/ 
/* foreach example 1: value only */ 
echo "foreach example 1: value only ".&#39;<br />&#39;; 
$a = array(1, 2, 3, 17); 
foreach ($a as $v) { 
echo "Current value of ".$a.":". $v."<br />"; 
} 
?> 
// 运行结果 
foreach example 1: value only 
Current value of $a: 1 
Current value of $a: 2 
Current value of $a: 3 
Current value of $a: 17

2. in_array,The meaning is to check the previous character Whether the string exists in the latter array, and in most cases, it works like this, but when the latter array is an integer, such as array(0,1,2,3), Something goes wrong, PHP will intval the previous string, and will get the value 0. So if by chance, your array has this value, then the equation is established. Is it beyond expectations?

So, when it is determined that the following data is an integer, especially if it may be 0 (this may replace all strings), you must not use this function anymore , you can use key_exists instead, but the subsequent data needs to be reversed using array_flip.

Let’s look at another example

function search($keyWord, $stack) {//此处判断是应该更新还是插入

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

        if (in_array($keyWord, $val)) {

          return TRUE;

        }

      }

      return FALSE;

    }

What happens when a string is used as an array to get values? PHP is a very fault-tolerant language. It will try its best to help you correct your mistakes, so it is very smart to convert your reference subscript into an integer. Of course, you will get 0. Then the string subscript is 0 and the value is You will get the value of the first string. Is it beyond your expectation? The solution is to confirm whether the variable is an array before referencing the subscript, is_array.

3. Problems encountered when referencing variables in foreach

I have been writing PHP for many years, but I still make stupid mistakes. Today I encountered oneforeach## Problem when referencing variable in #. The PHP version is 5.6.12. The code is as follows:

<?php
$arr = [&#39;a&#39;, &#39;b&#39;, &#39;c&#39;, &#39;d&#39;, &#39;e&#39;];
foreach ($arr as $i=>&$a) {
  $a = $a.&#39;_&#39;. $a;
  echo $a .&#39;<br>&#39;;
}
echo &#39;<hr>&#39;;
foreach ($arr as $i=>$a) {
  echo $a .&#39;<br>&#39;;
}
echo &#39;<hr>&#39;;
print_r($arr);

Output result

When I first saw the output result of the second foreach, I felt very confused. How could two d_d be output?

After thinking about it carefully, it turns out that the scope of $a in PHP foreach is the local variable of the entire

function, outside the loop is still valid, rather than being enclosed in a loop,

So when the second foreach is executed, $a is not a new variable, but still points to $arr

ArrayNo. 5 The address reference of the element,

When the second foreach is looping, it is actually constantly assigning values ​​to the fifth element of the $arr array,

Specific assignment situation,

The first time: a_a is assigned to the fifth element, the result is: [a_a, b_b, c_c, d_d,a_a]

The second time The third time: b_b is assigned to the 5th element, the result is: [a_a, b_b, c_c, d_d,b_b]

The third time: c_c is assigned to the 5th element, the result is: [a_a, b_b, c_c, d_d,c_c]

The fourth time: d_d is assigned to the fifth element, the result is: [a_a, b_b, c_c, d_d,d_d]

The fifth time: at this time Because the fifth element has become d_d and d_d is reassigned to the fifth element, the result is still: [a_a, b_b, c_c, d_d,d_d]

Solution:

1.尽量不使用相同的循环变量名；

2.每次使用完或再次使用前进行 unset($a); 处理，解除地址应用

还是用上面代码示例：

$arr = ['a', 'b', 'c', 'd', 'e'];
foreach ($arr as $i=>&$a) {
  $a = $a.'_'. $a;
  echo $a .&#39;<br>&#39;;
}
echo &#39;<hr>&#39;;
echo $a;
echo &#39;<hr>&#39;;

// 这里 unset 掉
unset($a);

echo $a = &#39;ccc&#39;;
echo &#39;<hr>&#39;;
print_r($arr);
echo &#39;<hr>&#39;;
foreach ($arr as $i=>$a) {
  echo $a .&#39;<br>&#39;;
}
echo &#39;<hr>&#39;;
print_r($arr);

输出结果：

现在就正常了，这些小细节一定要注意了。

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