Let's talk in depth about arrays in php

In PHP, array is a very common data structure. It can be used to store a series of data of the same type, such as integers, strings, Boolean values, etc. Compared with arrays in other programming languages, arrays in PHP have a very flexible implementation mechanism.

So, how are PHP arrays implemented? This article will introduce the implementation mechanism of PHP arrays, elaborating on the definition, internal structure, access methods, sorting algorithm, etc. of arrays.

1. Definition of array

In PHP, an array is a composite type that can store mixed values ​​of numeric, string, Boolean and other data types. Each element in the array is numbered in a certain order, and this number is called a "key value". In PHP, the definition of an array is very simple, as follows:

$array = array('foo', 'bar', 'baz');

This array contains 3 elements, which are the strings 'foo', 'bar', and 'baz'. In this array, the key values ​​of each element are 0, 1, and 2 in order.

2. Internal structure

PHP’s array uses a hash table (Hash Table) as its internal structure. The hash table is a very efficient data structure that can Enables fast data lookup and insertion operations. The key value of each array element is used as the index of the hash table, and the corresponding value is stored as the value of the hash table.

The internal implementation of the hash table is an array of buckets. Each bucket stores a linked list. The linked list nodes contain key values ​​and corresponding values. When operating on a hash table, the hash value is first calculated based on the key value, and then the linked list node is searched in the corresponding bucket. If the corresponding node is found, the value of this node can be directly manipulated, otherwise a new node needs to be inserted at the end of the linked list.

The advantage of hash tables is that for most data sets, the average time complexity of its query and insertion operations is O(1) level. Moreover, the hash table can also dynamically expand and contract, adaptively adjusting the size of the bucket array as the data set changes.

3. Access method

In PHP, we can access array elements through subscripts, as shown below:

echo $array[1];

This statement will output The 2nd element 'bar' in the array. PHP supports subscript access using array key values, for example:

$array['name'] = 'John';
echo $array['name'];

This statement will output the element 'John' with the key value 'name' in the array. It should be noted that the type of key value is not limited to string, it can be any data type.

PHP’s array also supports a series of commonly used operation methods, such as:

• array_push() – Push one or more elements to the end of the array;
• array_pop() – Pops and returns the last element of the array;
• array_shift() – Removes and returns the first element of the array;
• array_unshift() – At the beginning of the array Insert one or more elements;
• sort() – sort the array in ascending order;
• rsort() – sort the array in descending order;
• usort() – use Custom functions to sort arrays and more.

4. Sort algorithm

In PHP, array sorting can use the sort() function, rsort() function and usort() function. The sort() function and rsort() function are implemented through the quick sort algorithm, while the usort() function can be implemented using a user-defined sorting algorithm.

Quick sort algorithm is an efficient sorting algorithm. Its average time complexity is O(n log n), the worst-case time complexity is O(n^2), and the space complexity is is O(log n). The quick sort algorithm is divided into three steps:

1. Select the benchmark element;
2. Divide the array into two sub-arrays, one part is smaller than the benchmark element, and the other is larger than the benchmark element;
3. Sort subarrays recursively.

User-defined sorting algorithm can be implemented through the usort() function. Users need to write a comparison function themselves. This function accepts two elements as parameters and returns an integer representing their size relationship. For example:

function custom_sort($a, $b) {
    if ($a == $b) {
        return 0;
    } elseif ($a < $b) {
        return -1;
    } else {
        return 1;
    }
}

$array = array(4, 5, 1, 3, 2);
usort($array, "custom_sort");
print_r($array);

This code will output the sorted array: array(1, 2, 3, 4, 5).

Summary

PHP’s array is a very commonly used data structure. It uses a hash table as its internal structure to achieve fast data access and insertion operations. PHP's array also supports a variety of operation methods and sorting algorithms, providing a very flexible application method. Understanding the implementation mechanism of PHP arrays can help you better master PHP application development.

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