Storing key-value pairs with dictionaries

The last collection type we will look at is the dictionary. This is a familiar construct in programming languages, where it is sometimes referred to as a hash table. A dictionary holds a collection of pairings between a key and a value. The key can be any element that conforms to the Hashable protocol (just like elements in a set), while the value can be any type. The contents of a dictionary is not stored in order, unlike an array; instead, the key is used both when storing a value and as a lookup when retrieving a value.