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Friday, January 30, 2015

A Dictionary of Arrays in Swift

I was working through a raywenderlich.com tutorial and wrote this class as part of an Uber Challenge.  The challenge called for a random joke generator where the backing store was a Dictionary of Arrays.  In Swift, typing is loose, so there are many shorthand ways to express this data structure.  Here is one way

Swift is Apple's new iOS programming language.  You can define data structures explicitly.  See the generics-style definition in Swift familiar to Java developers.

    var favoriteSodas : Dictionary<String, Array<String>>

This can be replaced with the following shorthand

    var favoriteSodas : [String: [String]]

"favoriteSodas" is a Dictionary of Arrays.  The key is a String.

Defining Data Structures

Swift dispenses with the separate header and implementation files of Objective-C.  I initialize my favoriteSodas data structure in an init() method which is like a Java constructor

class FavoriteSodaGenerator {
    var favoriteSodas : [String: [String]]

    init() {
        favoriteSodas = [String: [String]]()

The values of the favoriteSodas Dictionary are Arrays.  Here's how I define three Arrays.

class FavoriteSodaGenerator {
    var favoriteSodas : [String: [String]]

    var carlSodas : [String]
    var janeSodas : [String]
    var daveSodas : [String]
    init() {
        favoriteSodas = [String: [String]]()
        carlSodas = [String]()
        carlSodas.append("ginger ale")
        janeSodas = [String]()
        janeSodas.append("diet pepsi")
        janeSodas.append("7 up")
        daveSodas = [String]()
        daveSodas.append("dr pepper")

I link the Arrays to the dictionary using the following block of code which closes out the init() method.

        favoriteSodas["carl"] = carlSodas
        favoriteSodas["jane"] = janeSodas
        favoriteSodas["dave"] = daveSodas

Accessing Arrays

In the challenge, a function is required to return a random item.  Based on an input person, this function returns a random item from one of the Arrays in the Dictionary.   If not found, and error is returned.

    func randomFavoriteSoda(person : String) -> String {
        if let sodas_a = favoriteSodas[person]  {
            let randomIndex = Int(arc4random()) % sodas_s.count
            return sodas_a[randomIndex]
        } else {
            return "Unrecognized person \(person)"


Finally, in my AppDelegate, I have the following code that tests the function and data structures.

        let favoriteSodaGenerator = FavoriteSodaGenerator()
        for index in 0 ..< {
            let f = favoriteSodaGenerator.randomFavoriteSoda("carl")
            println("Random carl favorite \(index): \(f)")

If you've been coding a while, the shorthand syntax won't seem like a big savings.  I suspect for beginners or developers coming from a Javascript background, the ability to initialize items with a minimum of syntax will be more appreciated.

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