round double to 0.5

Question

i have a result "1.444444" and i want to round this result to "1.5" this is the code i use :

a.text = String(round(13000 / 9000.0))

but this is round to "1.0" and i need round to "1.5"

and this code :

a.text = String(ceil(13000 / 9000.0))

is round to "2.0"

Multiply by 2, gives 1.8888.... Round, gives 3.0. Divide by 2, gives 1.5. — phoog, Dec 25 '15 at 07:22
Exactly. `floor()` and `ceil()` take you towards the nearest `.0` in each direction. If you want to end at `.5`, you need to divide an odd number by `2.0`. — Nicolas Miari, Dec 25 '15 at 07:24
@NicolasMiari and i can do it? phoog and if this other number its always give me this result? — Tzahi, Dec 25 '15 at 07:26
The comments by `phoog` and `Andrew Morton` provide the right answer. My comment is not completely correct, on second thought. — Nicolas Miari, Dec 25 '15 at 07:34
If you double your original number, what used to be half integer intervals (0.0, 0.5, 1.0, 1.5...) is expanded into whole number intervals (0.0, 1.0, 2.0,...). So, after doubling and rounding, your number is guaranteed to be in a whole integer boundary; half that and now it is only guaranteed to be on a half integer boundary. Does it make sense? — Nicolas Miari, Dec 25 '15 at 07:36
Try to picture a number line that stretches into double its size. — Nicolas Miari, Dec 25 '15 at 07:36

atxe · Answer 1 · 2017-06-15T16:50:43.960

50

Swift 3:

extension Double {
    func round(nearest: Double) -> Double {
        let n = 1/nearest
        let numberToRound = self * n
        return numberToRound.rounded() / n
    }

    func floor(nearest: Double) -> Double {
        let intDiv = Double(Int(self / nearest))
        return intDiv * nearest
    }
}

let num: Double = 4.7
num.round(nearest: 0.5)      // Returns 4.5

let num2: Double = 1.85
num2.floor(nearest: 0.5)     // Returns 1.5

Swift 2:

extension Double {
    func roundNearest(nearest: Double) -> Double {
        let n = 1/nearest
        return round(self * n) / n
    }
}

let num: Double = 4.7
num.roundNearest(0.5)      // Returns 4.5

edited Jun 15 '17 at 16:50

answered Jul 20 '16 at 13:44

atxe

5,029
2
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The Swift 3 example is great thank you, but please could you tell me how i could change this to always Round Down? i.e.. let num: Double = 1.85 num.round(nearest: 0.5) // Returns 2.0 but i want to return 1.5!!! – GameDev Jun 13 '17 at 13:16
that would be another question... but I updated my answer ;-) – atxe Jun 15 '17 at 16:51
That's great thank you, but in the end I did manage to work it out once I discovered the floor function :) – GameDev Jun 17 '17 at 01:11

Mark Lakata · Accepted Answer · 2015-12-26T20:19:54.567

x = 13000 / 9000.0;

denominator = 2;
a.text = String(round(x*denominator )/denominator );

First convert 1.444 to 2.888, then round to 3.0 and then divide by 2 to get 1.5. In this case, the denominator of 0.5 is 2 (i.e. 1/2). If you want to round to nearest quarter (0.25,0.5, 0.75, 0.00), then denominator=4

I Should point out that this works perfectly if the denominator is a power of 2. If it is not, say denominator=3, then you can get weird answers like 1.99999999 instead of 2 for particular values.

round double to 0.5

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