How to use array_udiff function to perform array difference in PHP (you can customize the comparison function)

In PHP, array is a very commonly used data type, and it is also very important for the operation and processing of arrays. When processing arrays, it is often necessary to perform operations such as comparing and taking differences between multiple arrays.

PHP provides many functions for arrays, among which the array_udiff() function is a function involving array differences. It can compare multiple arrays and return a difference array. This article will introduce in detail how to use the array_udiff() function and how to customize the comparison function to perform array difference.

1. Basic usage of array_udiff() function

The basic syntax of array_udiff() function is as follows:

array array_udiff ( array $array1 , array $array2 [, array $... ], callable $value_compare_func )

Among them, $array1, $array2 and subsequent $.. The parameters are multiple arrays to be compared, and $value_compare_func is a custom comparison function.

The return value of this function is an array, indicating the values ​​in the $array1 array that are different from other arrays.

The following is a simple example that demonstrates how to use the array_udiff() function:

<?php
function compare($a, $b){
    if($a === $b){ //值相等
        return 0;
    }
    return ($a > $b) ? 1 : -1;
}

$array1 = array(1, 2, 3, 4, 5);
$array2 = array(5, 6, 7, 8, 9);

$result = array_udiff($array1, $array2, 'compare');
print_r($result);
?>

The above code will output an array $result containing 1, 2, 3, and 4. These values Exists in $array1, but does not exist in $array2. This result is the same as we imagined, because arrays $array1 and $array2 only have the same value of 5, so the difference set should be all elements in $array1 except 5.

2. Implementation of custom comparison function

When using the array_udiff() function, you need to customize a comparison function, which is used to compare whether two elements are the same.

The comparison function definition needs to meet the following conditions:

1. The return value of the function must be an integer value;

2. The integer value represents two elements The size relationship, specifically:

   • If $a is greater than $b, the function should return a positive integer (a number greater than 0), such as 1;
   • If $a Less than $b, the function should return a negative integer (a number less than 0), such as -1;
   • If $a is equal to $b, the function should return 0.

The comparison function can be written according to the actual situation. For example, you can use the following function to compare the length of two strings:

function compare_length($a, $b){
    $len_a = strlen($a);
    $len_b = strlen($b);
    if($len_a === $len_b){ //值相等
        return 0;
    }
    return ($len_a > $len_b) ? 1 : -1;
}

In the above compare_length () function, we use the strlen() function to obtain the lengths of $a and $b, then compare the two lengths and return the corresponding value.

Of course, the comparison function is not necessarily only related to the value of the element. It can also be compared based on other attributes, such as a certain attribute of the object contained in the array, etc. This should be written according to the actual situation.

After customizing the comparison function, we can use the array_udiff() function to perform array difference. For example, we have the following two arrays:

$array1 = array('hello', 'world', 'foo', 'bar');
$array2 = array('world', 'bar', 'php');

If we want to find the elements in $array1 that are different from $array2, we can use the following code:

function compare_in_array($a, $b){
    if(in_array($a, $b)){ //值在另一个数组中存在
        return 0;
    }
    return ($a > $b) ? 1 : -1;
}

$result = array_udiff($array1, $array2, 'compare_in_array');
print_r($result);

Since in the comparison function compare_in_array( ), it is determined whether $a exists in the $b array. Therefore, elements with the same value as $array2 will be excluded from the array difference set.

3. Conclusion

The array_udiff() function can conveniently compare and difference multiple arrays. By customizing the comparison function, we can define the comparison method of elements based on our own needs. , handle arrays more flexibly.

It should be noted that when customizing a comparison function, it must meet the requirement that its return value is an integer, and the actual comparison logic must be written as needed.

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