Dictionary Basics / .keys / .values
A Swift dictionary (Dictionary) is a collection that stores key-value pairs. Keys are unique, and the order of elements is not guaranteed.
Syntax
// Declaring and initializing a dictionary var dict: [String: Int] = ["apple": 100, "banana": 200] var emptyDict = [String: Int]() // Empty dictionary var emptyDict2: [String: Int] = [:] // Empty dictionary (alternative syntax) // Accessing an element (returns an Optional) let value = dict["apple"] // Optional(100) // Listing keys and values let keys = dict.keys // Keys view let values = dict.values // Values view
Properties and Methods
| Property / Method | Description |
|---|---|
| dict[key] | Returns the value for the specified key as an Optional. |
| dict.count | Returns the number of key-value pairs in the dictionary. |
| dict.isEmpty | Returns a Bool indicating whether the dictionary is empty. |
| dict.keys | Returns all keys in the dictionary. |
| dict.values | Returns all values in the dictionary. |
| dict[key] = value | Sets a value for the given key. Assigning nil removes the key. |
Sample Code
// Creating a dictionary and performing basic operations
var prices: [String: Int] = ["apple": 100, "banana": 200, "cherry": 300]
// Retrieving a value (Optional)
if let price = prices["apple"] {
print("Price of apple: \(price)")
}
// Using a default value when the key does not exist
let grapePrice = prices["grape", default: 0]
print("Price of grape: \(grapePrice)")
// Adding and updating values
prices["grape"] = 400
prices["apple"] = 120
// Iterating over key-value pairs
for (fruit, price) in prices {
print("\(fruit): \(price)")
}
// Displaying keys in sorted order
let sortedKeys = prices.keys.sorted()
print("Keys: \(sortedKeys)")
// Dictionary information
print("Count: \(prices.count)")
print("Is empty: \(prices.isEmpty)")
// Checking if a key exists
if prices.keys.contains("banana") {
print("banana exists")
}
Overview
Swift dictionaries are declared with the type [Key: Value]. Accessing an element returns an Optional, so accessing a key that does not exist is safe and will not cause a crash.
You can use subscript access with a default value (dict[key, default: value]) to safely retrieve a value even when the key is absent. Because dictionaries do not guarantee key order, the iteration order when using for-in may differ each time.
For updating and deleting dictionary entries, see dict.updateValue() / Removing entries / mapValues().
If you find any errors or copyright issues, please contact us.
シェアする