edu.princeton.cs.algs4

Class PatriciaST<Value>

Object

edu.princeton.cs.algs4.PatriciaST<Value>

public class PatriciaST<Value>
extends Object

The PatriciaST class provides an implementation of an unordered symbol table of key-value pairs, with the restriction that the key is of class String. It supports the usual put, get, contains, delete, size, and is-empty methods. It also provides a keys method for iterating over all of the keys. A symbol table implements the associative array abstraction: when associating a value with a key that is already in the symbol table, the convention is to replace the old value with the new value. Unlike Map, this class uses the convention that values cannot be null—setting the value associated with a key to null is equivalent to deleting the key from the symbol table.

This unordered symbol table class implements PATRICIA (Practical Algorithm to Retrieve Information Coded In Alphanumeric). In spite of the acronym, string keys are not limited to alphanumeric content. A key may possess any string value, except for the string of zero length (the empty string).

Unlike other generic symbol table implementations that can accept a parameterized key type, this symbol table class can only accommodate keys of class String. This unfortunate restriction stems from a limitation in Java. Although Java provides excellent support for generic programming, the current infrastructure somewhat limits generic collection implementations to those that employ comparison-based or hash-based methods. PATRICIA does not employ comparisons or hashing; instead, it relies on bit-test operations. Because Java does not furnish any generic abstractions (or implementations) for bit-testing the contents of an object, providing support for generic keys using PATRICIA does not seem practical.

PATRICIA is a variation of a trie, and it is often classified as a space-optimized trie. In a classical trie, each level represents a subsequent digit in a key. In PATRICIA, nodes only exist to identify the digits (bits) that distinguish the individual keys within the trie. Because PATRICIA uses a radix of two, each node has only two children, like a binary tree. Also like a binary tree, the number of nodes, within the trie, equals the number of keys. Consequently, some classify PATRICIA as a tree.

The analysis of PATRICIA is complicated. The theoretical wost-case performance for a get, put, or delete operation is O(N), when N is less than W (where W is the length in bits of the longest key), and O(W), when N is greater than W. However, the worst case is unlikely to occur with typical use. The average (and usual) performance of PATRICIA is approximately ~lg N for each get, put, or delete operation. Although this appears to put PATRICIA on the same footing as binary trees, this time complexity represents the number of single-bit test operations (under PATRICIA), and not full-key comparisons (as required by binary trees). After the single-bit tests conclude, PATRICIA requires just one full-key comparison to confirm the existence (or absence) of the key (per get, put, or delete operation).

In practice, decent implementations of PATRICIA can often outperform balanced binary trees, and even hash tables. Although this particular implementation performs well, the source code was written with an emphasis on clarity, and not performance. PATRICIA performs admirably when its bit-testing loops are well tuned. Consider using the source code as a guide, should you need to produce an optimized implementation, for another key type, or in another programming language.

Other resources for PATRICIA:
Sedgewick, R. (1990) Algorithms in C, Addison-Wesley
Knuth, D. (1973) The Art of Computer Programming, Addison-Wesley

Author:

John Hentosh (based on an implementation by Robert Sedgewick)

Constructor Summary

Constructors

Constructor and Description
PatriciaST() Initializes an empty PATRICIA-based symbol table.

Method Summary

Modifier and Type	Method and Description
boolean	contains(String key) Returns true if the key-value pair, specified by the given key, exists within the symbol table.
void	delete(String key) Removes a key and its associated value from the symbol table, if it exists.
Value	get(String key) Retrieves the value associated with the given key.
Iterable<String>	keys() Returns all keys in the symbol table as an Iterable.
static void	main(String[] args) Unit tests the PatriciaST data type.
void	put(String key, Value val) Places a key-value pair into the symbol table.

Methods inherited from class Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

PatriciaST

public PatriciaST()

Initializes an empty PATRICIA-based symbol table.

Method Detail

put

public void put(String key,
                Value val)

Places a key-value pair into the symbol table. If the table already contains the specified key, then its associated value becomes updated. If the value provided is null, then the key becomes removed from the symbol table.

Parameters:

key - the key

val - the value

Throws:

IllegalArgumentException - if key is null

IllegalArgumentException - if key is the empty string.

get

public Value get(String key)

Retrieves the value associated with the given key.

Parameters:

key - the key

Returns:

the value associated with the given key if the key is in the symbol table and null if the key is not in the symbol table

Throws:

IllegalArgumentException - if key is null

IllegalArgumentException - if key is the empty string.

delete

public void delete(String key)

Removes a key and its associated value from the symbol table, if it exists.

Parameters:

key - the key

Throws:

IllegalArgumentException - if key is null

IllegalArgumentException - if key is the empty string.

contains

public boolean contains(String key)

Returns true if the key-value pair, specified by the given key, exists within the symbol table.

Parameters:

key - the key

Returns:

true if this symbol table contains the given key and false otherwise

Throws:

IllegalArgumentException - if key is null

IllegalArgumentException - if key is the empty string.

keys

public Iterable<String> keys()

Returns all keys in the symbol table as an Iterable. To iterate over all of the keys in the symbol table named st, use the foreach notation: for (Key key : st.keys()).

Returns:

all keys in the symbol table as an Iterable

main

public static void main(String[] args)

Unit tests the PatriciaST data type. This test fixture runs a series of tests on a randomly generated dataset. You may specify up to two integer parameters on the command line. The first parameter indicates the size of the dataset. The second parameter controls the number of passes (a new random dataset becomes generated at the start of each pass).

Parameters:

args - the command-line arguments