When accessing a dictionary, such as [String: Any], the result type is Optional(Any).
When indexing an array of [Any], the result type is Any, and the call can throw a fatal error.
Is there any reason for this difference?
It would be so nice to branch execution with a guard let, if let, ?, and ??, but instead you have to wrap array indexing in an if data.count <= index.
It's ultimately for performance reasons:
Commonly Rejected Changes
...
Strings, Characters, and Collection Types
- Make
Array<T>subscript access returnT?orT!instead ofT: The current array behavior is intentional, as it accurately reflects the fact that out-of-bounds array access is a logic error. Changing the current behavior would slowArrayaccesses to an unacceptable degree. This topic has come up multiple times before but is very unlikely to be accepted.
Though nothing stops you from rolling your own:
extension Collection {
subscript(safelyIndex i: Index) -> Element? {
get {
guard self.indices.contains(i) else { return nil }
return self[i]
}
}
}
let array = Array(0...10)
let n = array[safelyIndex: 3]
print(n as Any) // => Optional(3)
throw, it raises an exception. An out-of-range exception cannot be caught withtry - catch.subscript (index: Index) -> Element?method, but then you have to call it aslet b: Int? = a[0]orlet b = a[0] as Int?so that the compiler can distinguish the methods from the context. I would not recommend that. – Actually I would not use a “safe subscript” at all. As said above, you always know which indices are valid. A failure might indicate a logic error in your code.