Introduction to the usage of usort() function in PHP function library

PHP is a widely used programming language with many commonly used functions for developers to use. usort() is one of them. The main function of usort() is to sort the array according to the specified comparison function.

usort() function’s syntax is as follows:

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

In the above code, $array represents the array to be sorted, and $cmp_function is a Callable comparison function. The return value is a Boolean type, indicating whether the sorting was successful.

The following is a detailed introduction to the usort() function.

Comparison function

usort() The comparison function passed in the function must be callable, that is, a function that can be called and executed. Comparison functions are generally defined when calling usort(), either through an anonymous function or by specifying the function name. For example, you can define a comparison function like this:

function cmp($a, $b) {
    if ($a == $b) {
        return 0;
    }
    return ($a < $b) ? -1 : 1;
}

In this comparison function, $a and $b respectively represent the values ​​passed in by the usort() function to be compared. of two parameters. According to your own needs, you can customize the implementation method of the comparison function. The comparison function return value can be 0, -1, or 1. If 0 is returned, it means the two parameters are equal; if -1 is returned, it means $a is smaller than $b; if 1 is returned, it means $a is smaller than $b Big.

Usage

Next, let’s take a look at how to use the usort() function to sort the array. Suppose we have an array containing the following elements:

$array = [3, 1, 4, 1, 5, 9, 2, 6, 5, 3];

If we want to sort this array, we can use the usort() function:

usort($array, "cmp");

Here"cmp" is the name of the comparison function we defined. After running the above code, you can get the sorted array:

[1, 1, 2, 3, 3, 4, 5, 5, 6, 9]

usort() function can not only sort numbers, but also strings, objects, etc. Imagine we have an array containing multiple objects, each with an attribute value:

$students = [
    ["name" => "Alice", "age" => 18],
    ["name" => "Bob", "age" => 20],
    ["name" => "Charlie", "age" => 19],
];

If we want to sort these students by age, we can use the usort() function and define a new comparison Function:

function cmp_age($a, $b) {
    if ($a["age"] == $b["age"]) {
        return 0;
    }
    return ($a["age"] < $b["age"]) ? -1 : 1;
}

In this comparison function, we sort by comparing the age attribute of the student object. Use the usort() function to sort this student array:

usort($students, "cmp_age");

After running the above code, we can sort the students according to their age from small to large:

[
    ["name" => "Alice", "age" => 18],
    ["name" => "Charlie", "age" => 19],
    ["name" => "Bob", "age" => 20],
]

Notes

When using the usort() function, you need to pay attention to the following issues:

1. The comparison function passed in must be callable, that is, a function that can be called and executed.
2. The return value of the comparison function must be 0, -1 or 1.
3. usort() function will directly modify the original array, so when using it, you need to pay attention to whether the array will be referenced and used elsewhere.
4. When there are two or more equal elements in the array, the usort() function cannot guarantee their order.

Summary

usort() function is a very commonly used function in PHP development, it can sort arrays conveniently. When using it, we need to define a comparison function and pass it as a parameter to the usort() function. There are some details to be aware of, but overall, using the usort() function is very simple.

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