Swift Array - Check if an index exists

Add some extension sugar:

extension Collection {
  subscript(safe index: Index) -> Iterator.Element? {
    guard indices.contains(index) else { return nil }
    return self[index]
  }
}
if let item = ["a", "b", "c", "d"][safe: 3] { print(item) } // Output: "d"
// or with guard:
guard let anotherItem = ["a", "b", "c", "d"][safe: 3] else {return}
print(anotherItem) // "d"

Enhances readability when doing if let style coding in conjunction with arrays

Just check if the index is less than the array size:

if 2 < arr.count {
    ...
} else {
    ...
}

An elegant way in Swift:

let isIndexValid = array.indices.contains(index)

Type extension:

extension Collection {

    subscript(optional i: Index) -> Iterator.Element? {
        return self.indices.contains(i) ? self[i] : nil
    }

}

Using this you get an optional value back when adding the keyword optional to your index which means your program doesn't crash even if the index is out of range. In your example:

let arr = ["foo", "bar"]
let str1 = arr[optional: 1] // --> str1 is now Optional("bar")
if let str2 = arr[optional: 2] {
    print(str2) // --> this still wouldn't run
} else {
    print("No string found at that index") // --> this would be printed
}