Swift: Adding new key-value pairs to a dictionary

There are two ways to add new key values to a given dictionary in Swift. Let’s explore them in this concise example-based article.

Using the subscript syntax

You can use the subscript syntax to add a new key-value pair to a dictionary:

// this dictionary stores the products and their prices
var products = [String: Int]()

products["computer"] = 1000
products["mouse"] = 25

print(products)

Output:

["computer": 1000, "mouse": 25]

If you use the subscript syntax with a key that already exists, its value will be updated.

Using the updateValue(_:forKey:) method

The name of this method contains the word “update,” but it can still be used to add a new key-value pair to a dictionary. The method returns the old value if the key exists or nil if a new key-value pair is added.

Example:

var animals = ["cat": "meow", "dog": "woof"]
if let oldValue = animals.updateValue("moo", forKey: "cow") {
    print("The old value of \(oldValue) was replaced with a new one.")
} else {
    print("A new key-value pair was added to the dictionary.")
    print(animals)
}

Output:

A new key-value pair was added to the dictionary.
["cat": "meow", "cow": "moo", "dog": "woof"]