Collections: Dictionaries

This tutorial belongs to the Swift series

We use dictionaries to create a collection of key-value pairs.

Here is how to create a dictionary with 1 key-value pairs, where the key is a String and the value is an Int:

var dict = ["Roger": 8, "Syd": 7]

In this case the type is inferred. You can also explicitly set the type at declaration time:

var dict: [String: Int] = ["Roger": 8, "Syd": 7]

In this example we create an empty dictionary of Int keys and String values:

var dict = [String: Int]()

//or

var dict: [String: Int] = [:]

You can access the value assigned to a key using this syntax:

var dict = ["Roger": 8, "Syd": 7]

dict["Roger"] //8
dict["Syd"] //7

You can change the value assigned to a key in this way:

dict["Roger"] = 9

A dictionary must be declared as var to be modified. If it’s declared with let, you cannot modify it by adding or removing elements.

Use the same syntax to add a new key/value pair:

dict["Tina"] = 4

To remove a key/value paid, assign the value to nil:

dict["Tina"] = nil

Or call the removeValue(forKey:) method:

dict.removeValue(forKey: "Tina")

To get the number of items in the dictionary, use the count property:

var dict = ["Roger": 8, "Syd": 7]
dict.count //2

If a dictionary is empty, its isEmpty property is true.

var dict = [String: Int]()
dict.isEmpty //true

There are a lot of methods related to dictionaries, but those are the basic ones.

Dictionaries are passed by value, which means if you pass it to a function, or return it from a function, the dictionary is copied.

Dictionaries are collections, and they can be iterated over in loops.