Swift ?? operator, Characters

// 1. ?? operator
var a : Int?
let c : Int?
var b : Int = 123

var ss = a ?? b
print(ss)
ss = c ?? b // error : constant c is used before being initialized.
print(ss)
a = nil
ss = a ?? b
print(ss)




// 2. Working with Characters
for character in "Dog!^^".characters {
    print(character)
}



let catCharacters: [Character] = ["C","a","t","!"]
let catString = String(catCharacters)
print(catString)

var tmp = catString
tmp.append("?")
print(tmp)