PHP usort() function usage guide: sorting arrays

PHP usort() function usage guide: sorting arrays

In PHP programming, we often need to sort arrays. PHP provides many functions for sorting arrays, among which the usort() function can flexibly customize arrays. This article will introduce the usage and precautions of the usort() function, and demonstrate through examples how to use the usort() function to sort an array.

1. Introduction to usort() function

PHP usort() function is a powerful and flexible sorting function, which can sort arrays according to user-defined comparison functions. If you need to sort in ascending or descending order, you can use PHP's built-in sort function, but if you need to sort according to specific properties or rules of the array elements, the usort() function is very useful.

The syntax of the usort() function is as follows:

bool usort ( array &$array , callable $cmp_function )

The $array parameter represents the array to be sorted, and the $cmp_function parameter is a callback function used to specify the sorting rule. The syntax of the callback function is as follows:

int cmp_function ( mixed $a , mixed $b )

The callback function should return an integer value, which represents the size relationship between $a and $b. Returns an integer value greater than 0 if $a is greater than $b; returns an integer value less than 0 if $a is less than $b; returns 0 if $a is equal to $b.

2. Usage example of usort() function

The following is an example to demonstrate how to use the usort() function to sort an array. Suppose there is an array $employees, which contains information about multiple employees. Each employee contains three attributes: id, name and salary. Now we need to sort the employees array in descending order by salary.

$employees = array(
    array('id'=>101, 'name'=>'张三', 'salary'=>5000),
    array('id'=>102, 'name'=>'李四', 'salary'=>8000),
    array('id'=>103, 'name'=>'王五', 'salary'=>6000),
    array('id'=>104, 'name'=>'赵六', 'salary'=>7000),
);

function cmp($a, $b) {
    if ($a['salary'] == $b['salary']) {
        return 0;
    }
    return ($a['salary'] > $b['salary']) ? -1 : 1;
}

usort($employees, 'cmp');

print_r($employees);

Run the above code, you will get the following output:

Array (
    [0] => Array ( [id] => 102 [name] => 李四 [salary] => 8000 )
    [1] => Array ( [id] => 104 [name] => 赵六 [salary] => 7000 )
    [2] => Array ( [id] => 103 [name] => 王五 [salary] => 6000 )
    [3] => Array ( [id] => 101 [name] => 张三 [salary] => 5000 )
)

As can be seen from the above results, we use the usort() function to sort the array $employees in descending order, and the final result is Also in line with expectations.

3. Notes

Although the usort() function is very flexible and powerful, there are some things you need to pay attention to when using it.

First of all, the array elements to be used in the callback function must exist, otherwise an error will occur. Therefore, before using the usort() function, you must ensure that all elements to be used in the array have been initialized.

Secondly, the callback function should be as simple and fast as possible, otherwise it will affect the efficiency of the program. Since each sorting requires calling the callback function, the efficiency of the callback function has a great impact on the speed of sorting. When writing callback functions, you should try to avoid using a large number of loops and judgment statements, as well as complex calculations.

Finally, when the number of array elements is large, the efficiency of the sorting algorithm will also be affected. In this case, we can consider using other sorting functions such as PHP's built-in sort() and rsort() functions.

4. Summary

This article briefly introduces the usage and precautions of the PHP usort() function, and gives an example to demonstrate how to use this function to sort an array. Using the usort() function can flexibly and conveniently customize the sorting of the array, but when writing callback functions, pay attention to efficiency and algorithm complexity to avoid affecting the performance of the program.

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