Rounding Numbers to Two Significant Figures

Question

I am sure this is an easy question to any of you are experienced in Swift, however, I just started learning how to program and have no idea where to start. What I am trying to do is a round a number to the nearest whole value, or to the third number. This is what I mean:

12.6 //Want rounded to 13
126 //Want rounded to 130
1264 //Want rounded to 1300

I know swift has a .rounded() function, and I have managed to use it to round the nearest 10th, 100th, etc., however, I cannot round the way I would like to. Any advice would be much appreciated.

Answer 1

Here's one way to round any Double or Int (including negative numbers) to a given number of significant figures:

func round(_ num: Double, to places: Int) -> Double {
    let p = log10(abs(num))
    let f = pow(10, p.rounded() - Double(places) + 1)
    let rnum = (num / f).rounded() * f

    return rnum
}

func round(_ num: Int, to places: Int) -> Int {
    let p = log10(abs(Double(num)))
    let f = pow(10, p.rounded() - Double(places) + 1)
    let rnum = (Double(num) / f).rounded() * f

    return Int(rnum)
}

print(round(0.265, to: 2))
print(round(1.26, to: 2))
print(round(12.6, to: 2))
print(round(126, to: 2))
print(round(1264, to: 2))

Output:

0.27
1.3
13.0
130
1300