MultiDictionary

A MultiDictionary is a data structure that allows a single key to be associated with multiple values. Unlike a standard dictionary where each key maps to a unique value, a MultiDictionary permits several values to be stored under the same key. This is often implemented using a hash map where the value associated with each key is a list or set of elements.

The primary advantage of a MultiDictionary is its ability to efficiently store and retrieve collections of related data. For example, one could use a MultiDictionary to store all the words that rhyme with a particular word, with the rhyming word as the key and the list of rhyming words as the values. Similarly, it can be used to map a category to all the items belonging to that category.

Common operations on a MultiDictionary include adding a value to a key, retrieving all values associated with a key, and removing a specific value from a key. The underlying implementation details can vary, but the conceptual model remains consistent: a one-to-many mapping. Many programming languages provide built-in or library support for MultiDictionary-like structures, often referred to by different names such as a map of lists, multimap, or grouped dictionary. The choice of using a list or a set for storing values depends on whether duplicate values for a single key are permitted and whether the order of values is important.