Dictionary creation and access in Swift

Introduction

Dictionaries help you store pairs of related information, like names and phone numbers. You can quickly find a value by using its key.

When you want to store a list of items with labels, like a phone book.

When you need to look up information fast using a unique key.

When you want to group related data together, like a person's details.

When you want to update or add new information easily by key.

Syntax

Swift

var dictionaryName: [KeyType: ValueType] = [key1: value1, key2: value2]

// Access value by key
let value = dictionaryName[key]

Keys must be unique and hashable (like strings or numbers).

Accessing a key returns an optional value because the key might not exist.

Examples

This creates a dictionary with names as keys and ages as values. Then it gets Alice's age.

Swift

var ages: [String: Int] = ["Alice": 30, "Bob": 25]
let aliceAge = ages["Alice"]

Swift can infer the types. This dictionary stores countries and their capitals.

Swift

var capitals = ["France": "Paris", "Japan": "Tokyo"]
let capitalOfJapan = capitals["Japan"]

This creates an empty dictionary and adds a key-value pair later.

Swift

var emptyDict = [String: String]()
emptyDict["key"] = "value"

Sample Program

This program creates a dictionary of fruits and their colors. It safely accesses the color of Apple and Orange, showing how to handle missing keys.

Swift

import Foundation

var fruitsColors: [String: String] = ["Apple": "Red", "Banana": "Yellow", "Grape": "Purple"]

if let appleColor = fruitsColors["Apple"] {
    print("The color of Apple is \(appleColor).")
} else {
    print("Apple color not found.")
}

// Trying to access a key that does not exist
if let orangeColor = fruitsColors["Orange"] {
    print("The color of Orange is \(orangeColor).")
} else {
    print("Orange color not found.")
}

Important Notes

Always use optional binding (if let) when accessing dictionary values to avoid errors if the key is missing.

You can add or update values by assigning to a key, like dict["newKey"] = newValue.

Dictionaries do not keep order, so the order of items is not guaranteed.

Summary

Dictionaries store data in key-value pairs for quick lookup.

Use square brackets with a key to get or set values.

Accessing a key returns an optional, so handle missing keys safely.