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Swift by Example: Dictionaries

Swift 5.x

Understanding Swift dictionaries with this code example showing key-value pair storage, optional value access, safe unwrapping with if-let, adding and updating entries, removing keys by setting nil, and iterating over dictionary contents.

Code

// Creating a dictionary
var airports: [String: String] = ["YYZ": "Toronto Pearson", "DUB": "Dublin"]

// Accessing values (returns an Optional)
if let airportName = airports["DUB"] {
    print("The name of the airport is (airportName).")
} else {
    print("That airport is not in the dictionary.")
}

// Adding/Updating
airports["LHR"] = "London Heathrow"
airports["YYZ"] = "Toronto Pearson International"

// Removing
airports["DUB"] = nil

// Iterating
for (code, name) in airports {
    print("(code): (name)")
}

Explanation

Dictionaries store associations between keys and values. Each key must be unique. Like arrays, dictionaries are strongly typed. The syntax [KeyType: ValueType] is used to define the type.

Accessing a dictionary using a key returns an Optional value. This is because the key might not exist in the dictionary. You should safely unwrap the result using if let or provide a default value using the nil-coalescing operator ??.

You can add or update a value by assigning to a key using subscript syntax. Assigning nil to a key removes that key-value pair from the dictionary. Dictionaries are unordered, so iterating over them does not guarantee any specific order.

Code Breakdown

2
[String: String] defines a dictionary with string keys and string values.
5
if let airportName = airports["DUB"] safely unwraps the optional value.
12
airports["LHR"] = "London Heathrow" adds a new key-value pair.
16
airports["DUB"] = nil removes the key-value pair from the dictionary.
ArraysClasses