Dictionaries

A dictionary stores key-value pairs: you look up a value by its key instead of by position.

Real-world analogy: a dictionary is a contacts list — you find a phone number by the person's name, not by where it is in the list.

Syntax

var ages = ["Ada": 36, "Linus": 28]      // [String: Int]
var empty: [String: Double] = [:]

Reading values

Looking up a key returns an optional, because the key might not exist:

let ages = ["Ada": 36, "Linus": 28]

if let age = ages["Ada"] {
    print("Ada is \(age)")
}
print(ages["Grace"] == nil)
print(ages["Grace", default: 0])
print(ages.count)

Ada is 36
true
0
2

ages["Grace", default: 0] returns a plain Int — the default when the key is missing.

Changing a dictionary

var stock = ["apples": 10, "pears": 4]

stock["plums"] = 7             // add
stock["apples"] = 12           // update
stock["pears"] = nil           // remove
stock["apples", default: 0] += 1

for fruit in stock.keys.sorted() {
    print("\(fruit): \(stock[fruit, default: 0])")
}

apples: 13
plums: 7

Order is not guaranteed

Dictionaries are unordered — looping over one directly can give the keys in any order, which may change from run to run. Sort the keys (as above) when order matters for output.

let prices = ["tea": 3, "cake": 4]

for (item, price) in prices.sorted(by: { $0.key < $1.key }) {
    print("\(item): $\(price)")
}

cake: $4
tea: $3

Common mistakes

• Using a dictionary value without unwrapping it: ages["Ada"] + 1 doesn't compile — the lookup is an Int?.
• Expecting the printed order to match the order you wrote.

Practice

Count how many times each word appears in ["red", "blue", "red", "green", "red"] using a dictionary, and print the counts in alphabetical order.

let words = ["red", "blue", "red", "green", "red"]
var counts: [String: Int] = [:]

for word in words {
    counts[word, default: 0] += 1
}

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

blue: 1
green: 1
red: 3