Using iOS NumberFormatter to format decimal into fraction representation

Question

In iOS, how can use NumberFormatter to format the decimal part of a number into its fractional representations.

For example I would like to format the number 2.375 as 2 3/8.

Can this be done with NumberFormatter?

Answer 1

No. You would need to implement your own Rational Formatter. You can use this answer from Martin R as a starting point. You can do something like:

class RationalFormatter: Formatter {
    let precision: Double = 1.0E-6
    override public func string(for obj: Any?) -> String? {
        guard let value = obj as? Double else { return nil }
        let whole = modf(value).0
        var x = modf(value).1
        var a = x.rounded(.down)
        var (h1, k1, numerator, denominator) = (1, 0, Int(a), 1)
        while x - a > precision * Double(denominator) * Double(denominator) {
            x = 1.0/(x - a)
            a = x.rounded(.down)
            (h1, k1, numerator, denominator) = (numerator, denominator, h1 + Int(a) * numerator, k1 + Int(a) * denominator)
        }
        var string = ""
        if whole < 0 || numerator < 0 {
            string += "-"
        }
        if whole != 0 {
            string += String(Int(abs(whole)))
        }
        if whole != 0 && numerator != 0 {
            string += " "
        }
        if numerator != 0 {
            string += "\(abs(numerator))/\(abs(denominator))"
        }
        return string
    }
}

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Usage:

let double = 2.375
let rationalFormatter = RationalFormatter()
let formatted = rationalFormatter.string(for: double)   // "2 3/8"