Collections: arrays, dictionaries, sets

In a music app a playlist keeps songs in a certain order, a phone book finds a number by a name, and a “liked” list cannot contain the same photo twice. For these three situations Swift has three collections: the array (Array), the dictionary (Dictionary) and the set (Set).

Arrays

An array is an ordered list of values of the same type, written in square brackets. Every element has an index, and counting starts at 0: the first element is fruits[0]. An array's type is written like [String] or [Int]. An array declared with var can be changed; a let array cannot.

var fruits = ["apple", "banana", "cherry"]
print(fruits)
print(fruits[0])
print(fruits.count)

fruits.append("pear")
fruits.insert("kiwi", at: 1)
print(fruits)

fruits.remove(at: 2)
print(fruits)
print(fruits.contains("pear"))
print(fruits.first ?? "empty")

["apple", "banana", "cherry"]
apple
3
["apple", "kiwi", "banana", "cherry", "pear"]
["apple", "kiwi", "cherry", "pear"]
true
apple

first and last return optionals, because an array can be empty. To loop over an array use for-in, and if you need both the index and the value, use enumerated(). sorted() returns a new sorted array, and max() and min() return the largest and smallest element (as optionals).

let scores = [85, 92, 78]
for (index, score) in scores.enumerated() {
    print("\(index + 1). \(score)")
}
print(scores.sorted())
print(scores.max() ?? 0)

let empty: [Int] = []
print(empty.isEmpty)

1. 85
2. 92
3. 78
[78, 85, 92]
92
true

Dictionaries

A dictionary stores key: value pairs and finds a value by its key, not by an index. Its type is written like [String: String]. A value read by a key is always optional, because the key might not exist — this is where ?? is very useful. Assigning nil to a key removes the pair.

var capitals = ["Azerbaijan": "Baku", "Georgia": "Tbilisi"]
capitals["Japan"] = "Tokyo"

print(capitals["Azerbaijan"] ?? "unknown")
print(capitals["France"] ?? "unknown")
print(capitals["Japan"])

capitals["Georgia"] = nil
print(capitals.count)

for country in capitals.keys.sorted() {
    print("\(country) -> \(capitals[country] ?? "")")
}

Baku
unknown
Optional("Tokyo")
2
Azerbaijan -> Baku
Japan -> Tokyo

One of the most useful jobs for a dictionary is counting. A class voted for its favourite animal — how many votes did each animal get?

let votes = ["cat", "dog", "cat", "fish", "cat", "dog"]
var counts: [String: Int] = [:]

for vote in votes {
    counts[vote, default: 0] += 1
}

for animal in counts.keys.sorted() {
    print(animal, counts[animal, default: 0])
}

cat 3
dog 2
fish 1

Sets

A set (Set) is a collection of unique values with no order. If you add the same value twice, it is stored only once. Sets check very quickly whether an element is present and support intersection (intersection) and union (union), just like in mathematics.

var visited: Set = ["Baku", "Ganja", "Baku", "Shaki"]
print(visited.count)

visited.insert("Lankaran")
print(visited.contains("Ganja"))

let friendVisited: Set = ["Baku", "Quba"]
print(visited.intersection(friendVisited).sorted())
print(visited.union(friendVisited).sorted())

3
true
["Baku"]
["Baku", "Ganja", "Lankaran", "Quba", "Shaki"]

`map`, `filter` and `reduce`

There are three powerful methods for processing collections without writing a loop. map transforms every element into a new array, filter keeps only the elements that match a condition, and reduce combines all elements into one value. The code in curly braces is a closure, and $0 is the current element — you will study closures in detail in the next lesson.

let prices = [4, 12, 7, 20]

let doubled = prices.map { $0 * 2 }
let cheap = prices.filter { $0 < 10 }
let total = prices.reduce(0, +)

print(doubled)
print(cheap)
print(total)
print(prices.filter { $0 >= 10 }.reduce(0, +))

let names = ["aysel", "murad"]
print(names.map { $0.uppercased() })

[8, 24, 14, 40]
[4, 7]
43
32
["AYSEL", "MURAD"]