Swift Code Explained: Implementing Safe Array Subscription

When working with arrays in Swift, you might have come across a common error called “Fatal error: Index out of range”. This error typically occurs when you unintentionally try to access an index that is not present in the array. Actually, accessing elements from an array is not safe by default. Therefore, today we’re going to explore how to code more securely when accessing elements in an array. 😊

Let’s look at an example:

let shoppingList = ["apples", "raspberries"]
let item = shoppingList[2]

Accessing an element this way will crash the app if the element isn’t there.

We’ll define an extension on the Collection type that adds a “safe” subscript. This subscript returns an optional value, which will be nil if the index is out of bounds.

extension Collection {

    /// Returns the element at the specified index if it exists, otherwise nil.
    subscript (safe index: Index) -> Element? {
        return indices.contains(index) ? self[index] : nil
    }
}

By using “if let” statement to unwrap the optional value, we can safely access the array item without worrying about out-of-bounds errors

if let item = shoppingList[safe: 2] {
    // use item
}

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Thanks for Reading! ✌️

If you have any questions or corrections, please leave a comment below or contact me via my LinkedIn account Pham Trung Huy.

Happy coding 🍻