How to round a Double to the nearest Int in swift?

Question

I'm trying to make a calculator of growth rate (Double) that will round the result to the nearest Integer and recalculate from there, as such:

let firstUsers = 10.0
let growth = 0.1
var users = firstUsers
var week = 0


while users < 14 {
    println("week \(week) has \(users) users")
    users += users * growth
    week += 1
}

but I've been unable so far.

EDIT I kinda did it like so:

var firstUsers = 10.0
let growth = 0.1
var users:Int = Int(firstUsers)
var week = 0


while users <= 14 {
    println("week \(week) has \(users) users")
    firstUsers += firstUsers * growth
    users = Int(firstUsers)
    week += 1
}

Although I don't mind that it is always rounding down, I don't like it because firstUsers had to become a variable and change throughout the program (in order to make the next calculation), which I don't want it to happen.

Answer 1

There is a round available in the Foundation library (it's actually in Darwin, but Foundation imports Darwin and most of the time you'll want to use Foundation instead of using Darwin directly).

import Foundation

users = round(users)

Running your code in a playground and then calling:

print(round(users))

Outputs:

15.0

round() always rounds up when the decimal place is >= .5 and down when it's < .5 (standard rounding). You can use floor() to force rounding down, and ceil() to force rounding up.

If you need to round to a specific place, then you multiply by pow(10.0, number of places), round, and then divide by pow(10, number of places):

Round to 2 decimal places:

let numberOfPlaces = 2.0
let multiplier = pow(10.0, numberOfPlaces)
let num = 10.12345
let rounded = round(num * multiplier) / multiplier
print(rounded)

Outputs:

10.12

Note: Due to the way floating point math works, rounded may not always be perfectly accurate. It's best to think of it more of an approximation of rounding. If you're doing this for display purposes, it's better to use string formatting to format the number rather than using math to round it.