Why can't I divide integers in swift?

Question

In the Swift "Tour" documentation, there's an exercise where you build on the following function to average a set of numbers:

func sumOf(numbers: Int...) -> Int {     var sum = 0     for number in numbers {         sum += number     }     return sum } 

I can make this work using something like the following:

func averageOf(numbers: Double...) -> Double {     var sum: Double = 0, countOfNumbers: Double = 0     for number in numbers {         sum += number         countOfNumbers++     }     var result: Double = sum / countOfNumbers     return result } 

My question is, why do I have to cast everything as a Double to make it work? If I try to work with integers, like so:

func averageOf(numbers: Int...) -> Double {     var sum = 0, countOfNumbers = 0     for number in numbers {         sum += number         countOfNumbers++     }     var result: Double = sum / countOfNumbers     return result } 

I get the following error: Could not find an overload for '/' that accepts the supplied arguments

Answer 1

The OP seems to know how the code has to look like but he is explicitly asking why it is not working the other way.

So, "explicitly" is part of the answer he is looking for: Apple writes inside the "Language Guide" in chapter "The Basics" -> "Integer and Floating-Point Conversion":

Conversions between integer and floating-point numeric types must be made explicit

Answer 2

you just need to do this:

func averageOf(numbers: Int...) -> Double {     var sum = 0, countOfNumbers = 0     for number in numbers {         sum += number         countOfNumbers++     }     var result: Double = Double(sum) / Double(countOfNumbers)     return result }