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About Dictionaries

As dictionaries are one of Swift's three primary collection types, knowing how to work with dictionaries is a big part of knowing how to program in Swift.

Key Takeaways from this Exercise

• Dictionaries store mappings between keys which are elements of one type and values
• Values can be of any type.
• Keys can be of any type that conforms to the Hashable protocol. All of the basic Swift types have this property, and it can be added to other types.
• Dictionaries are unordered collections.
• Dictionaries can be defined using dictionary literals:

var addresses: Dictionary<String, String> = ["The Munsters": "1313 Mockingbird Lane", "The Simpsons": "742 Evergreen Terrace", "Buffy Summers": "1630 Revello Drive"]
var sequences: [String: [Int]] = ["Euler's totient": [1, 1, 2, 2, 4, 2, 6, 4], "Lazy caterer": [1, 2, 4, 7, 11, 16, 22, 29, 37], "Carmichael": [561, 1105, 1729, 2465, 2821, 6601, 8911, 10585, 15841]]
let constants: = ["pi": 3.14159, "e": 2.71828, "phi": 1.618033, "avogadro": 6.02214076e22]

• Empty dictionaries can be written by following the type name of the dictionary by a pair of parenthesis, e.g. [Int: String](), or, if the type can be determined from the context, as just a pair of square brackets surrounding a colon, [:].
• Type names for dictionaries are written in one of two ways: Dictionary<K, V> or [K: V] where K is the type of the keys in the dictionary and V is the type of the values.
• Dictionary elements can be accessed using subscript notation, e.g. addresses["The Simpsons"].
• Values returned from these look-ups are optionals; nil is returned if a requested key is not present in the dictionary.
• To avoid the optional type of the return, one can supply a default value to return if the key in not found in the dictionary.

let e: Double = constants["e", default: 0]
// => 2.71828
let hoagie: [Int] = sequences["Hoagie", default: []]
// => []
let betty = addresses["Betty Cooper", default: "Address unknown"]
// => "Address unknown"

• The subscript notation can be used to set the value associated with the supplied key, provided the dictionary was defined as a variable (i.e. using var).
• This can be used to update the value associated with a key or to add a new key: value pair to the dictionary.
• Dictionaries can be sorted by passing a sorting function into the dictionary's sorted(by:) method.
• For-in loops can be used to iterate through the key: value pairs of a dictionary
• Each pair is presented as a tuple and can be destructured like other tuples to assign each element to a name. For example, to print out the address book, one can write:

for (name, address) in addresses {
  print("\(name) lives at \(address)")
}

// prints out:
// Betty Cooper lives at 111 Queens Ave.
// The Munsters lives at 1313 Mockingbird Lane
// The Simpsons lives at 742 Evergreen Terrace
// Buffy Summers lives at 1630 Revello Drive

The other methods available for working with dictionaries can be seen in the Apple Swift API documentation.