How to write method to calculate average in Swift-playground

It's so much easier with just a straightforward call to reduce:

let array = [1.0,2.0,3.0]
var average = array.reduce(0.0) {
    return $0 + $1/Double(array.count)
}
// average = 2.0

You can use the count property of the array:

func average(numbers: Int...) -> Double {
    var sum = 0
    for number in numbers {
        sum += number
    }
    var  ave : Double = Double(sum) / Double(numbers.count)
    return ave
}

While others have already pointed out the missing count variable, it is worth noting that you can do the average even more elegantly by using closures as follows:

func averagec(numbers:Int...) -> Double {
    return Double(numbers.reduce(0,combine:+))/Double(numbers.count)
}

Edited for Swift 4:-

func averagec(numbers:Int...) -> Double {
    return Double(numbers.reduce(0,+))/Double(numbers.count)
}

import Accelerate

let avg = vDSP.mean(array)

This will be at least 50x faster than all other solutions posted so far.