The principle, implementation and common problems of PHP hash table

The hash table maps keys to array subscripts through hash functions to achieve fast search, insertion and deletion. PHP implements hash tables using arrays and the md5() hash function to resolve collisions through linear probing. Common problems include hash collisions (can be solved by increasing the array size or optimizing the hash function), hash collisions (can be avoided by secure hash functions), and performance (depends on the hash function and collision resolution method). Practical cases such as word counting, quickly counting word frequencies through hash tables.

The principle, implementation and common problems of PHP hash table

The principle of hash table

Hash table is a structure that maps keys to an array subscript through a hash function, which can quickly find, insert and delete data. It consists of the following components:

• Array: An array that stores elements.
• Hash function: A function that maps keys to array subscripts.
• Conflict resolution: A method to resolve conflicts when different keys map to the same subscript.

Hash table implementation in PHP

PHP uses arrays as hash tables. The hash function is PHP's md5() function, which converts a string into a unique 32-bit hash value.

Create and initialize hash table

$hashTable = [];

Insert data

$key = "key";
$value = "value";
$hashTable[$key] = $value;

Find data

$key = "key";
if (isset($hashTable[$key])) {
  $value = $hashTable[$key];
}

Delete data

$key = "key";
unset($hashTable[$key]);

Conflict resolution

PHP uses linear exploration conflict resolution, that is, when a conflict occurs, start from Ha Starting from the subscript returned by the hash function, the subscripts are incremented by 1 one by one until a free position is found.

FAQ

• Hash conflict: Occurs when different keys map to the same subscript, which can be achieved by increasing the array size Or use a better hash function to solve it.
• Hash collision: Occurs when different keys produce the same hash value, this is rare but can be avoided by using a secure hash function.
• Performance: The performance of a hash table is highly dependent on the quality of the hash function and collision resolution.

Practical case: word counting

Use a hash table to implement the word counting function:

function wordCount($text) {
  $hashTable = [];
  $words = explode(" ", $text);
  foreach ($words as $word) {
    if (isset($hashTable[$word])) {
      $hashTable[$word]++;
    } else {
      $hashTable[$word] = 1;
    }
  }
  return $hashTable;
}

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