stdev in Swift 2
This is an implementation of stdev in Swift 2
Swift introduced Protocol extension in Swift 2 so I though I would try to implement stdev() for [NSNumber]. I think this provides an elegant solution to a common problem. Sure, you still have to map your doubles/integers/… into NSNumber, it would be nice to be able to create stdev() for [Double] but I don't think it is already possible in Swift.
Usage
let doubles = [1.0, 2.0]
let numbers = doubles.map { NSNumber(double:$0) }
numbers.stdev