Swift Dictionaries

title: Dictionaries


A dictionary is a type of collection that stores an unordered list of associated “key-value” pairs. Each key is a unique identifier that stores a specified value, which then can be accessed or referenced later with the corresponding key.


Various ways to initialize a Dictionary:

//Full -> Dictionary<Key, Value> var emptyDictionary1: Dictionary<String, String> = Dictionary(); var emptyDictionary2 = Dictionary<Int,String>() //Shorthand -> [Key: Value] (preferred) var emptyDictionary3 = [Int: Float](); var emptyDictionary4: [Float: String] = [:]

(1) The shorthand variant(s) are the preferred style to use due to the brevity and clarity it provides in comparison to its “full” variations.

(2) You can utilize any type as the key of a dictionary as long as it conforms to the Hashable Protocol. For more information check out Protocols and Hashable.

Create many Entries using a “Dictionary Literal”.

A dictionary literal is a list of key-value pairs in the following shorthand form … [ key1: value1, key2: value2, key3: value3 ]

var pokeDexionary: [Int: String] = [ 25: "Pitachew", 10: "Caterpie", 17: "Pidgeotto", 1: "Bulbasaur", 4: "Charmander" ]

Accessing and Modifying

Getting Values

When getting the values from a dictionary it will return an optional value as it’s possible to not have a value associated with the key provided in the subscript.

let charmander = pokeDexionary[4] // Optional("Charmander") let trySquirtle = pokeDexionary[7] // returns nil... cause no one will trade with me!

Adding New Values

pokeDexionary[7] = "Squirtle" pokeDexionary[98] = "Krabby"

Updating Values

print(pokeDexionary[25]!) // outputs: "Pitachew" pokeDexionary[25] = "Pikachu" print(pokeDexionary[25]!) // outputs: "Pikachu"

“UpdateValue()” Method

You can also utilize the updateValue method to modify an entry or add a new one. The way it works is that if the key exists in the dictionary it updates the value for the key, otherwise it will add the key as a new entry with the given value. Also if updateValue adds a new entry it will return nil, but if the method modifies a current value then it will return the old value.

let nilValue = pokeDexionary.updateValue("RaticakeIsALie", forKey: 20); // nilValue = nil; let oldValue = pokeDexionary.updateValue("Raticate", forKey: 20) //oldValue = "RaticakeIsALie"

Removing a value from a dictionary

pokeDexionary[20] = nil; //removed "Raticate" ... pokeDexionary.updateValue("Butterfree", forKey: 12)// and added "Butterfree" instead


The most common way to iterate over a dictionary is by using a for-in loop.

for (key, value) in pokeDexionary { print("Who's That Pokemon? Its.... #\(key): \(value)") }

Output For Above:

Who’s That Pokemon? Its…. #4: Charmander.
Who’s That Pokemon? Its…. #10: Caterpie.
Who’s That Pokemon? Its…. #98: Krabby.
Who’s That Pokemon? Its…. #7: Squirtle.
Who’s That Pokemon? Its…. #1: Bulbasaur.
Who’s That Pokemon? Its…. #17: Pidgeotto.
Who’s That Pokemon? Its…. #25: Pikachu.

Note: Because dictionaries have no designated order the above output can be in different order each time you run it.

More Information:

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