Let's talk about the implementation principles of php arrays

PHP is a very popular programming language that is widely used in web development. A very important feature of PHP is its array. PHP's array is known as a powerful and flexible data structure. It can store various types of data, including strings, integers, floating point numbers, etc. So, how are PHP arrays implemented? Let’s find out below.

The Concept of PHP Arrays

Before starting to explore the implementation principles of PHP arrays, you first need to understand the concept of PHP arrays. In PHP, an array is an ordered data structure with corresponding key values. Elements in the array can be accessed through indexes or associated keys. Specifically, a PHP array can be defined as a series of elements, each element containing a key-value pair, where the key is the unique identifier used to access the element, and the value is the data item actually stored in the element.

For example, the following is a simple PHP array:

$students = array("Tom", "Jerry", "Spike");

In this array, $students is the array variable name, and "Tom", "Jerry", "Spike" are three elements in the array. These elements are arranged in order, and the position of each element can be accessed using an index, for example:

echo $students[0]; // 输出 "Tom"
echo $students[1]; // 输出 "Jerry"
echo $students[2]; // 输出 "Spike"

In addition, PHP's array also supports associated keys, that is, using strings as keys to access elements in the array element. For example:

$grades = array("Tom" => 85, "Jerry" => 90, "Spike" => 80);
echo $grades["Tom"]; // 输出 85
echo $grades["Jerry"]; // 输出 90
echo $grades["Spike"]; // 输出 80

The implementation principle of PHP array

PHP array is actually a data structure that implements a hash table. Hash table, also known as hash table, is an efficient data structure that can be used to implement data types such as dictionaries and sets. Its characteristic is that it can quickly find, insert and delete elements, and the time complexity is usually O(1).

In a hash table, the index of an element is calculated through the hash function. The hash function maps the key to a position in the array, which is the index of the element in the array. Since the hash function is an efficient calculation method, the hash table can quickly locate the location of the element.

In PHP, the implementation of arrays is based on hash tables. When creating an array, PHP allocates a memory space for the array and initializes a hash table structure to store the elements in the array. The structure of this hash table usually contains the following parts:

1. Array capacity (capacity): Indicates the space size for storing elements in the hash table;
2. Number of elements (size) : Indicates the number of elements stored in the hash table;
3. Load factor (load factor): Indicates the ratio of the occupied space to the total space in the hash table, used to determine whether the hash table needs to be expanded. Or shrink;
4. Hash function: used to calculate the index of elements in the hash table;
5. Conflict handling method: used to solve the situation where multiple elements are mapped to the same index position.

The hash function of PHP array calculates the index position based on the key of the element. Different keys will be mapped to different positions. For example, if we have an associative array $grades, which contains the grades corresponding to the three key values ​​​​of "Tom", "Jerry", and "Spike", PHP's hash function will be based on these three The values ​​(i.e. names) of the keys are used to calculate their index positions in the array.

The implementation of the hash function is usually to calculate the index by adding the ASCII code of the key and taking the modulo, for example:

$index = array_sum(str_split("Tom")) % $capacity;

In this way, the key value can be calculated as a unique The index value and points this index value to a location in the hash table. If multiple keys calculate the same index, a conflict will occur. The way PHP's hash table handles conflicts is to use a linked list to store conflicting elements.

When a conflict occurs, PHP will insert the element into the end of the linked list at the corresponding index, thus ensuring that different elements can be stored in the hash table. When looking for an element, PHP will calculate the corresponding index position based on the value of the key, and then search along the linked list corresponding to the index position until an element equal to the key value is found.

Expansion and contraction is a very important function of PHP arrays. When the load factor in the hash table exceeds a certain threshold, expansion is required to increase the capacity of the hash table. When the load factor in the hash table is too low, shrinkage is needed to reduce the capacity of the hash table. Expansion and contraction will cause certain performance overhead, so PHP will dynamically adjust the capacity of the hash table to achieve optimal performance.

Conclusion

PHP array is a powerful and flexible data structure based on hash table. It provides PHP developers with a convenient and efficient data processing method. By understanding the implementation principles of PHP arrays, we can better understand the application of array data structures in PHP.

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