Swift Dictionaries and Operations on Dictionaries

Dictionaries are collection types in Swift that allow you to store a set of key-value pairs. Each key in the dictionary must be unique, and can be of any hashable type, while the values can be of any type. In this article, we'll explore the basics of dictionaries in Swift and how to perform various operations on them.

Creating a Dictionary

To create an empty dictionary in Swift, you can use the Dictionary keyword, followed by the data types for the keys and values in angle brackets. You can also use the shorthand syntax [KeyType: ValueType] to create an empty dictionary of the specified types.

1var emptyDict: Dictionary<String, Int> = [:] // empty dictionary with String keys and Int values
2var anotherDict: [String: String] = [:] // shorthand syntax for empty dictionary with String keys and values

You can also create a dictionary with pre-populated key-value pairs by using the dictionary literal syntax. This involves enclosing the key-value pairs in square brackets and separating them with a colon.

1var colors = ["red": "#FF0000", "green": "#00FF00", "blue": "#0000FF"]

Accessing Dictionary Elements

To access the value associated with a particular key in a dictionary, you can use the subscript notation with the key as the index.

1let redHex = colors["red"] // "#FF0000"

You can also use optional binding to safely unwrap an optional value returned by the subscript operator.

1if let greenHex = colors["green"] {
2    print(greenHex) // "#00FF00"
3}

Adding and Updating Elements

You can add a new key-value pair to a dictionary by using the subscript notation with a new key and value. If the key already exists in the dictionary, the associated value will be updated with the new value.

1colors["yellow"] = "#FFFF00" // adds new key-value pair
2colors["red"] = "#FF3333" // updates existing value for "red" key

Removing Elements

You can remove a key-value pair from a dictionary by setting the value for the key to nil, or by using the removeValue(forKey:) method.

1colors["green"] = nil // removes key-value pair for "green" key
2let blueHex = colors.removeValue(forKey: "blue") // removes and returns value for "blue" key

Iterating Over a Dictionary

You can iterate over the key-value pairs in a dictionary using a for-in loop. The loop will iterate over each key-value pair in an arbitrary order.

1for (key, value) in colors {
2    print("\(key): \(value)")
3}

Alternatively, you can iterate over just the keys or values in a dictionary using the keys or values properties, respectively.

1for key in colors.keys {
2    print(key)
3}
4
5for value in colors.values {
6    print(value)
7}