Why can php arrays store dictionaries?

PHP is a commonly used server-side programming language, in which arrays are a very important data type. Unlike arrays in other programming languages, PHP arrays can store dictionaries. This article will explore the principles and methods of why PHP arrays can store dictionaries.

1. The basics of PHP arrays

In PHP, an array is a data structure that can store multiple values. Each value in the array has a unique key (key), and the corresponding value can be accessed through the key. There are three ways to declare arrays in PHP:

1. Use the array() function

$arr ​​= array('a', 'b', 'c' );

2. Use [] notation

$arr ​​= ['a', 'b', 'c'];

3. Use the list() function

list($a, $b, $c) = ['a', 'b', 'c'];

In PHP, The keys of the array can be integers or strings, while the values ​​of the array can be of any data type, such as integers, strings, floats, booleans, etc.

2. PHP arrays can store dictionaries

In PHP, in addition to storing basic types of values, arrays can also store dictionaries. The so-called dictionary is a set of key-value pairs, where each key corresponds to a unique value. For example:

$dict = ['name' => 'Zhang San', 'age' => 18, 'gender' => 'Male'];

The above code , $dict is an array of dictionaries where each key corresponds to a value. We can access the corresponding value through the key, such as:

echo $dict['name']; // Output: Zhang San

In addition, PHP also provides some array functions to operate Dictionary array, such as the following example:

$dict = ['name' => 'Zhang San', 'age' => 18, 'gender' => 'Male'];
$keys = array_keys($dict); // Get all keys
$values ​​= array_values($dict); // Get all values ​​
$hasName = array_key_exists('name', $dict); // Determine whether the specified key exists

As shown above, we can get all the keys in the dictionary array through the array_keys() function, get all the values ​​through the array_values() function, and determine whether it is through the array_key_exists() function The specified key exists.

3. Implementation Principle of PHP Dictionary Array

PHP’s dictionary array is actually a hash table (Hash Table), also called a hash table. A hash table is a data structure that directly accesses memory locations based on keywords, and is characterized by fast search and insertion operations.

In PHP, when we declare a dictionary array, the PHP engine automatically allocates memory space for it and uses a hash table to store the array elements. When we access an element of the dictionary array, the PHP engine will locate the corresponding memory address based on the hash value of the key and return the value stored in that address.

By using hash tables, PHP arrays can achieve O(1) complexity addition, deletion and search operations, which is one of the key reasons why it is widely used in practical applications.

4. Notes on Dictionary Arrays

Although PHP arrays can store dictionaries, you need to pay attention to the following points in practical applications:

1. Keys must be unique

In a PHP array, each key must be unique, otherwise earlier key values ​​will be overwritten by later key values. We can merge two dictionaries into one through the array_merge() function while removing duplicate key-value pairs.

2. The key cannot be a Boolean value, NULL, or an array

The key of a PHP array must be a string or an integer, and cannot be a Boolean value, NULL, or an array. If you try to use these types as key values, the PHP engine will automatically convert them to integers.

3. The order of keys can be changed

PHP The keys in the array are unordered. When we use the var_dump() function or the foreach loop to traverse the array, the order of the keys Subject to change. We can use the ksort() or krsort() function to sort the array in ascending or descending order by key name.

5. Conclusion

PHP arrays can store dictionaries, which is one of the main reasons why they are widely used in practical applications. By understanding the relevant knowledge and precautions of PHP arrays, we can have a deeper understanding of PHP's data types and hash table data structures.

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