Swift Dictionary: Syntax, Usage, and Examples

Dictionaries in Swift store key-value pairs, letting you map unique identifiers to specific data. They’re great for fast lookups, structured data organization, and counting or grouping information.

How to Define and Use a Dictionary

Create a dictionary using square brackets with a colon between key and value types:

var studentGrades: [String: Int] = ["Alice": 90, "Bob": 85, "Clara": 92]

You can also let Swift infer the types:

var capitals = ["France": "Paris", "Japan": "Tokyo", "India": "New Delhi"]

To create an empty dictionary:

var inventory: [String: Int] = [:]

Accessing and Updating Entries

Access a value by its key:

let grade = studentGrades["Alice"]  // Output: Optional(90)

Since the key might not exist, the return type is optional.

Update or insert a value:

studentGrades["Bob"] = 88      // Update
studentGrades["Diana"] = 91    // Insert

Remove an entry:

studentGrades["Alice"] = nil

When to Use a Dictionary

Dictionaries are ideal for pairing identifiers with data. Common use cases include:

1. App Settings and Preferences

let settings = ["theme": "dark", "fontSize": "medium"]

2. Mapping IDs to Data

let users = [101: "Emma", 102: "Noah", 103: "Liam"]

3. Grouping by Category

let names = ["Alice", "Aaron", "Bob", "Becky"]
let grouped = Dictionary(grouping: names, by: { $0.prefix(1) })
// ["A": ["Alice", "Aaron"], "B": ["Bob", "Becky"]]

4. Counting Occurrences

let words = ["apple", "banana", "apple"]
var count: [String: Int] = [:]

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

Examples

Create and Access Values

var countryCodes = ["US": "United States", "FR": "France", "JP": "Japan"]
print(countryCodes["FR"] ?? "Unknown")

Loop Through a Dictionary

let prices = ["apple": 1.2, "banana": 0.8, "cherry": 2.0]

for (fruit, price) in prices {
    print("\(fruit): $\(price)")
}

Check for Existence

if let value = countryCodes["JP"] {
    print("Found: \(value)")
} else {
    print("Key not found.")
}

Use Default Values

let population = ["NY": 8_336_817]
let bostonPop = population["Boston", default: 0]

Advanced Dictionary Concepts

Ordered Dictionaries

Standard dictionaries don’t guarantee key order. In practice, insertion order is preserved (since Swift 5.2), but you shouldn’t rely on it.

To guarantee order, use OrderedDictionary from Swift Collections:

import OrderedCollections

var orderedDict: OrderedDictionary = ["first": 1, "second": 2]
orderedDict["third"] = 3
print(orderedDict[0])  // 1

Dictionary vs. Array

Use a dictionary when you:

• Need fast key-based lookups
• Want to group or count items
• Require unique keys

Use an array when you:

• Care about order
• Need to store duplicate values
• Plan to iterate over all elements in sequence

Filtering and Transforming

You can filter by value:

let scores = ["Amy": 95, "Ben": 82, "Chris": 99]
let highScorers = scores.filter { $0.value > 90 }

Transform values:

let bonusScores = scores.mapValues { $0 + 5 }

Transform keys and values:

let doubled = Dictionary(uniqueKeysWithValues: scores.map { ($0.key, $0.value * 2) })

Merging Dictionaries

var config = ["theme": "light", "volume": "low"]
let override = ["theme": "dark"]

config.merge(override) { (_, new) in new }
// ["theme": "dark", "volume": "low"]

Nested Dictionaries

Useful for modeling structured or API-style data:

let library = [
    "fiction": ["1984": "Orwell", "Dune": "Herbert"],
    "nonfiction": ["Sapiens": "Harari"]
]

if let author = library["fiction"]?["Dune"] {
    print("Author of Dune: \(author)")
}

Optimizing Performance

If you expect to store many entries, reserve capacity to reduce reallocations:

var cache = [String: Int]()
cache.reserveCapacity(1000)

Converting to Arrays

Convert keys or values:

let sortedKeys = prices.keys.sorted()
let valuesArray = prices.values.map { $0 }

Sort by value:

let sortedByPrice = prices.sorted { $0.value < $1.value }

Summary

Dictionaries let you store and retrieve values efficiently using unique keys. They’re a go-to tool for counting, grouping, mapping, and organizing data in Swift. You can access, mutate, remove, and filter data with concise syntax and solid performance.

For more advanced scenarios, you can work with ordered dictionaries, nested structures, or use mapValues and filter for transformation. Swift’s dictionary type is both powerful and flexible—making it a core part of any well-structured app.