Swift3, extension to mutablizate a String array

Question

I believe you make an extension on a [String] like this...

extension Sequence where Iterator.Element == String {

Say I want to mutate the array - I mean to say, change each of the strings in the array. How to do?

extension Sequence where Iterator.Element == String {
   func yoIzer() {
      for s in self { s = "yo " + s }
   }
}

That does not work.

(That's just an example, more complex processing may be required: you may wanna avoid just using a filter.)

Answer 1

A Sequence is not mutable, and in any case changing the element s would not change anything about the Sequence it comes from (s is a copy).

What you are trying to say is this:

extension MutableCollection where Iterator.Element == String {
    mutating func yo() {
        var i = self.startIndex
        while i != self.endIndex {
            self[i] = "yo" + self[i]
            i = self.index(after: i)
        }
    }
}

And here's a test:

var arr = ["hey", "ho"]
arr.yo()
print(arr)
// ["yohey", "yoho"]

That approach actually comes straight out of the Swift docs.