How do I incrementally rotate through an array with Swift? - arrays

Question

Just learning swift and wanted to rotate through an array of colors like so:
class ColorSwitcher
{
let colors:String[] = ["red", "blue", "green"]
var currIndex:Int?
var selectedColor:String{
return self.colors[currIndex!]
}
init(){
currIndex = 0
}
func changeColor()
{
currIndex++ //this doesn't work
}
}
When I try to call the function like so:
var switcher:ColorSwitcher = ColorSwitcher()
switcher.selectedColor // returns red
switcher.changeColor()
switcher.selectedColor // still returns red
The problem is with the changeColor function. The error I get is:
Could not find an overload for '++' that accepts the supplied arguments
What am I doing wrong?

Answer 1

The problem is that currIndex is an optional. I'd suggest refactoring like so:
class ColorSwitcher {
let colors:String[] = ["red", "blue", "green"]
var currIndex:Int = 0
var selectedColor:String {
return self.colors[currIndex]
}
func changeColor() {
currIndex++
}
}
if you want to keep it an optional you'll need to do this:
currIndex = currIndex! + 1
but of course that isn't safe, so you should probably do:
if let i = currIndex {
currIndex = i + 1
}
else {
currIndex = 1
}
Also, keep in mind that you don't need to use an optional if you're going to set the value in your init(). The following is fine:
class ColorSwitcher {
let colors:String[] = ["red", "blue", "green"]
var currIndex:Int
init(startIndex: Int) {
currIndex = startIndex
}
var selectedColor:String {
return self.colors[currIndex]
}
func changeColor() {
currIndex++
}
}

Answer 2

you can overload the missing ++ operator for optional Int, like e.g. this:
#assignment #postfix func ++(inout x: Int?) -> Int? {
if x != nil {
x = x! + 1
return x
} else {
return nil
}
}
or you can change your class, like e.g. this:
class ColorSwitcher {
let colors:String[] = ["red", "blue", "green"]
var currIndex: Int = 0
var selectedColor: String {
return self.colors[currIndex]
}
func changeColor() {
currIndex++
}
}
NOTE: that does not hold any improvement in your class's internal behaviour. it will do the same for you as did in your OP.