find the standard deviation of all elements in an array in swift

main.swift
func standardDeviation(_ arr: [Double]) -> Double? {
    let count = arr.count
    guard count > 1 else { return nil } // there must be at least 2 elements to calculate the standard deviation
    
    let sum = arr.reduce(0, +)
    let mean = sum / Double(count)
    
    let squaredDifferencesSum = arr.reduce(0) { result, element in
        let difference = element - mean
        return result + difference * difference
    }
    
    return sqrt(squaredDifferencesSum / Double(count - 1))
}

let arr = [1.0, 2.0, 3.0, 4.0, 5.0]
if let sd = standardDeviation(arr) {
    print("Standard deviation of \(arr) is \(sd)") // Standard deviation of [1.0, 2.0, 3.0, 4.0, 5.0] is 1.5811388300841898
} else {
    print("Array must have at least 2 elements")
}
753 chars
22 lines