Dictionaries

Swift only provides two collection types. One is arrays and the other is dictionaries. Each value in a dictionary is associated with a unique key. If you’re familiar with NSDictionary in Objective C, the syntax of initializing a dictionary in Swift is similar. Here is an example:

Objective C:

NSDictionary *companies = @{@"AAPL" : @"Apple Inc", @"GOOG" : @"Google Inc", @"AMZN" :@"Amazon.com, Inc", @"FB" : @"Facebook Inc"};

Swift:

var companies = ["AAPL" : "Apple Inc", "GOOG" : "Google Inc", "AMZN": "Amazon.com, Inc",
"FB" : "Facebook Inc"]

The key and value in the key-value pairs are separated by a colon. Like array and other variables, Swift automatically detects the type of the key and value. However, if you like, you can specify the type information by using the following syntax:

var companies: Dictionary<String, String> = ["AAPL" : "Apple Inc", "GOOG" : "Google Inc",
"AMZN" : "Amazon.com, Inc", "FB" : "Facebook Inc"]

To iterate through a dictionary, use the for-in loop.

for (stockCode, name) in companies {
    println("\(stockCode) = \(name)")
}

You can also use the keys and values properties to retrieve the keys and values of the dictionary.

for stockCode in companies.keys {
    println("Stock code = \(stockCode)")
}
for name in companies.values {
    println("Company name = \(name)")
}

To access the value of a particular key, specify the key using the subscript syntax. If you want to add a new key-value pair to the dictionary, simply use the key as the subscript and assign it with a value like below:

companies["TWTR"] = "Twitter Inc"

Now the “companies” dictionary contains a total of 5 items. The “TWTR”:”Twitter Inc” pair is automatically added to the “companies” dictionary.