swift generics return first and last element - swift

Question

I'm trying to get used to generics (never used them in objc) and want to write a toy function that takes an object of any type () and returns the first and last element. Hypothetically, I'd only use this on an array or a string - I keep getting an error that has no subscript members. I totally understand that the error message is telling me swift has no clue that T may potentially hold a type that does have subscripts - I just want to know how to get around this.
func firstAndLastFromCollection<T>(a:T?) {
var count: Int = 0
for item in a as! [AnyObject] {
count++
}
if count>1 {
var first = a?[0]
var last = a?[count-1]
return (first, last)
}
return something else here
}
Do I need to typecast somewhere here (which would kind of defeat the purpose here, as I'd need to downcast as either a string or an array, adding code and lessening how generic this func is)?

Answer 1

If you want to return the first and the last element then it's probably safe assuming the input param is an array of some kind of type.
So you can implement your function this way
func firstAndLast<T>(list:[T]) -> (first:T, last:T)? {
guard let first = list.first, last = list.last else { return nil }
return (first, last)
}
The function does return a tuple of 2 element, both have the same type of the generic element of the input array.
The returned tuple is an option because if the array is empty then nil is returned.
Examples
let nums = firstAndLast([1,2,3,4])
let words = firstAndLast(["One", "Two", "Three"])
As you can verify the type of the generic element into the array becomes the type of the elements inside the tuple.
In the example above nums is inferred to be (Int, Int)? and words (Words, Words)?
More examples
let emptyList: [String] = []
firstAndLast(emptyList) // nil
Extension
Finally you can also write this code as an extension of Array.
extension Array {
var firstAndLast: (first:Element, last:Element)? {
guard let first = self.first, last = self.last else { return nil }
return (first, last)
}
}
Now you can write
let aCoupleOfShows = ["Breaking Bad", "Better Call Saul", "Mr Robot"].firstAndLast
Again, if you check the type of the constant aCoupleOfShows you'll see that is a (first: String, last: String)?. Swift automatically did infer the correct type.
Last example
In the comments you said you wanted the first and last chars of a String. here it is the code if you use the extension above
if let chars = Array("Hello world".characters).firstAndLast {
print("First char is \(chars.first), last char is \(chars.last) ")
}
//>> First char is H, last char is d

Answer 2

If we are talking about collections, let's use the CollectionType:
func firstAndLastFromCollection<T: CollectionType>(a: T) -> (T.Generator.Element, T.Generator.Element)? {
guard !a.isEmpty else {
return nil
}
return (a.first!, a.lazy.reverse().first!)
}
print(firstAndLastFromCollection(["a", "b", "c"])) // ("a", "c")
print(firstAndLastFromCollection("abc".characters)) // ("a", "c")
print(firstAndLastFromCollection(0..<200)) // (0, 199)
print(firstAndLastFromCollection([] as [String])) // nil
If you specify your generic type to also conform to bidirectional index:
func firstAndLastFromCollection<T: CollectionType where T.Index : BidirectionalIndexType>(...) -> ...
then you can call last directly:
return (a.first!, a.last!)
If we decide to implement it using a category, we don't need generics at all:
extension CollectionType {
func firstAndLast() -> (Generator.Element, Generator.Element)? {
guard !self.isEmpty else {
return nil
}
return (self.first!, self.lazy.reverse().first!)
}
}
extension CollectionType where Index: BidirectionalIndexType {
func firstAndLast() -> (Generator.Element, Generator.Element)? {
guard !self.isEmpty else {
return nil
}
return (self.first!, self.last!)
}
}
print("abc".characters.firstAndLast())
Swift is a protocol oriented language. Usually you will find yourself extend protocols more than extending classes or structs.