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Improved dictionary functionality

Available from Swift 4.0

Paul Hudson      @twostraws

One of the most intriguing proposals for Swift 4 was to add some new functionality to dictionaries to make them more powerful, and also to make them behave more like you would expect in certain situations.

Let's start with a simple example: filtering dictionaries in Swift 3 does not return a new dictionary. Instead, it returns an array of tuples with key/value labels. For example:

let cities = ["Shanghai": 24_256_800, "Karachi": 23_500_000, "Beijing": 21_516_000, "Seoul": 9_995_000];
let massiveCities = cities.filter { $0.value > 10_000_000 }

After that code runs you can't read massiveCities["Shanghai"] because it is no longer a dictionary. Instead, you need to use massiveCities[0].value, which isn't great.

As of Swift 4 this behaves more like you would expect: you get back a new dictionary. Obviously this will break any existing code that relies on the tuple-array return type.

Similarly, the map() method on dictionaries never quite worked the way many people hoped: you got a key-value tuple passed in, and could return a single value to be added to an array. For example:

let populations = { $0.value * 2 }

That hasn't changed in Swift 4, but there is a new method called mapValues() that is going to be much more useful because it lets you transform the values and place them back into a dictionary using the original keys.

For example, this code will round and stringify all city populations, then put them back into a new dictionary with the same keys of Shanghai, Karachi, and Seoul:

let roundedCities = cities.mapValues { "\($0 / 1_000_000) million people" }

(In case you were wondering, it's not safe to map dictionary keys because you might create duplicates by accident.)

Easily my favorite new dictionary addition is a grouping initializer, which converts a sequence into a dictionary of sequences that are grouped by whatever you want. Continuing our cities example, we could use cities.keys to get back an array of city names, then group them by their first letter, like this:

let groupedCities = Dictionary(grouping: cities.keys) { $0.characters.first! }

That will output the following:

["B": ["Beijing"], "S": ["Shanghai", "Seoul"], "K": ["Karachi"]]

Alternatively, we could group the cities based on the length of their names like this:

let groupedCities = Dictionary(grouping: cities.keys) { $0.count }

That will output the following:

[5: ["Seoul"], 7: ["Karachi", "Beijing"], 8: ["Shanghai"]]

Finally, it's now possible to access a dictionary key and provide a default value to use if the key is missing:

let person = ["name": "Taylor", "city": "Nashville"]
let name = person["name", default: "Anonymous"]

Now, any experienced developer will probably argue that's better written using nil coalescing, and I agree. You could write this line instead using the current version of Swift:

let name = person["name"] ?? "Anonymous"

However, that doesn't work when you're modifying the dictionary value rather than just reading it. You can't modify a dictionary value in place because accessing its key returns an optional – the key might not exist, after all. With Swift 4's default dictionary values you can write much more succinct code, such as this:

var favoriteTVShows = ["Red Dwarf", "Blackadder", "Fawlty Towers", "Red Dwarf"]
var favoriteCounts = [String: Int]()

for show in favoriteTVShows {
    favoriteCounts[show, default: 0] += 1

That loops over every string in favoriteTVShows, and uses a dictionary called favoriteCounts to keep track of how often each item appears. We can modify the dictionary in one line of code because we know it will always have a value: either the default value of 0, or some higher number based on previous counting.

For more information see the Swift Evolution proposal for these new features.

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