A simple function that calculates linear regression lines in Swift

LinearRegression

//
//  main.swift
//  LinearRegression
//
//  Created by Thomas Ciha on 5/20/17.
//  Copyright © 2017 Thomas Ciha. All rights reserved.
//

import Foundation

//write an algorithm that calculates a linear regression line for a set of data

func calculateLine(data: Double...) -> String{
    if(data.count % 2 != 0){
        return "Unable to calculate, missing a y-value"
    }
    var x = 0.0, y = 0.0, xy = 0.0, x2 = 0.0, m : Double, index = 0
    
    for value in data{
        if(index % 2 == 0){ //if even index, it's an x-value
            x += value
            xy += value * data[index + 1]
            x2 += value * value
        }
        else { //if odd index, it's a y-value
            y += value
        }
        index += 1
    }
    let num_of_xy_pairs = Double(data.count / 2)
    x /= num_of_xy_pairs
    y /= num_of_xy_pairs
    xy /= num_of_xy_pairs
    x2 /= num_of_xy_pairs
    m = round((x * y - xy) / ((x * x) - x2) * 1000) / 1000
    let b = round((y - m * x) * 1000) / 1000
    
    if(b < 0){
        return "y = \(m)x \(b)"
    }
    else{
        return "y = \(m)x + \(b)"
    }
}

print(calculateLine(data: 17,23,123,7,4,6,9,10,11,12))