One thing to note: aside from programming interviews, these operations are fairly rare. And that's a good thing, because none of these produce results that are very intuitive, because they are not very well defined on strings in general (I don't fault Swift for this, but it's just a general problem with text). Using any of these to create a new String may cause entirely new characters to show up, or the length of the text to change. So Swift actually doesn't expose these as "string" operations, but operations on the characters themselves; in each case returning a new collection of characters that is not a String. Now, you can reconstitute them into a String pretty easily, but you should keep the this in mind when doing so.
How would you safely get the nth index of a string, clamped to the valid indexes? So if the nth index is out-of-bounds you get the first/last index instead?
extension String {
func character(atClampedIndex index: Int) -> Character? {
guard !self.isEmpty else {
return nil
}
let clamped = max(0, min(index, count - 1))
return self[self.index(startIndex, offsetBy: clamped)]
}
}
Swift is pretty pedantically strict about Unicode correctness, and avoids some pitfalls which lead to incorrect handling (e.g., integer-based indexing). This means that the traditional way many developers might be used to interacting with strings is cumbersome and annoying — but once you get past the initial hurdle*, most operations are actually (1) really easy, especially when expressed in terms of generic Sequence/Collection operations, and (2) much more difficult to get wrong.
*I think the largest part of that hurdle is overcoming what you may have gotten used to from other languages, i.e., treating strings as an array of "characters", for some language/library definition of "character" (whether bytes, code points, etc.). It's relatively rare that you actually care about indexing into an arbitrary spot in a string: instead, combinations of slicing operations (including `prefix(_:)`, `dropFirst(_:)`, `take(while:)`, etc.) and generic Collection operations will get you what you want. Things like `.reversed()`, `.sorted(), and `.shuffled()` all work trivially correctly too (since you're not operating on a bag of bytes), and it's exceedingly rare that user input will confound the operations you might need to perform. (Exception: operations like case folding and collection, which are locale-specific, need special handling through a framework like Foundation.)
To be clear: not everything is sunshine and roses, but an amazing amount of functionality "falls out" of basic protocol conformances on String, and its exposure as a Collection of grapheme clusters.
Given a specific string manipulation task, I'd be happy to provide an example of what it might look like in Swift!