Precision String Format Specifier In Swift

Question

Below is how I would have previously truncated a float to two decimal places

NSLog(@\" %.02f %.02f %.02f\", r, g, b);

I checked the docs an

Answer 1

You can also create an operator in this way

operator infix <- {}

func <- (format: String, args:[CVarArg]) -> String {
    return String(format: format, arguments: args)
}

let str = "%d %.1f" <- [1453, 1.123]

Answer 2

Swift2 example: Screen width of iOS device formatting the Float removing the decimal

print(NSString(format: "Screen width = %.0f pixels", CGRectGetWidth(self.view.frame)))

Answer 3

Details

• Xcode 9.3, Swift 4.1
• Xcode 10.2.1 (10E1001), Swift 5

Solution 1

func rounded() -> Double

(5.2).rounded()
// 5.0
(5.5).rounded()
// 6.0
(-5.2).rounded()
// -5.0
(-5.5).rounded()
// -6.0

func rounded(_ rule: FloatingPointRoundingRule) -> Double

let x = 6.5

// Equivalent to the C 'round' function:
print(x.rounded(.toNearestOrAwayFromZero))
// Prints "7.0"

// Equivalent to the C 'trunc' function:
print(x.rounded(.towardZero))
// Prints "6.0"

// Equivalent to the C 'ceil' function:
print(x.rounded(.up))
// Prints "7.0"

// Equivalent to the C 'floor' function:
print(x.rounded(.down))
// Prints "6.0"

mutating func round()

var x = 5.2
x.round()
// x == 5.0
var y = 5.5
y.round()
// y == 6.0
var z = -5.5
z.round()
// z == -6.0

mutating func round(_ rule: FloatingPointRoundingRule)

// Equivalent to the C 'round' function:
var w = 6.5
w.round(.toNearestOrAwayFromZero)
// w == 7.0

// Equivalent to the C 'trunc' function:
var x = 6.5
x.round(.towardZero)
// x == 6.0

// Equivalent to the C 'ceil' function:
var y = 6.5
y.round(.up)
// y == 7.0

// Equivalent to the C 'floor' function:
var z = 6.5
z.round(.down)
// z == 6.0

Solution 2

extension Numeric {

    private func _precision(number: NSNumber, formatter: NumberFormatter) -> Self? {
        if  let formatedNumString = formatter.string(from: number),
            let formatedNum = formatter.number(from: formatedNumString) {
                return formatedNum as? Self
        }
        return nil
    }

    private func toNSNumber() -> NSNumber? {
        if let num = self as? NSNumber { return num }
        guard let string = self as? String, let double = Double(string) else { return nil }
        return NSNumber(value: double)
    }

    func precision(_ minimumFractionDigits: Int,
                   roundingMode: NumberFormatter.RoundingMode = NumberFormatter.RoundingMode.halfUp) -> Self? {
        guard let number = toNSNumber() else { return nil }
        let formatter = NumberFormatter()
        formatter.minimumFractionDigits = minimumFractionDigits
        formatter.roundingMode = roundingMode
        return _precision(number: number, formatter: formatter)
    }

    func precision(with numberFormatter: NumberFormatter) -> String? {
        guard let number = toNSNumber() else { return nil }
        return numberFormatter.string(from: number)
    }
}

Usage

_ = 123.44.precision(2)
_ = 123.44.precision(3, roundingMode: .up)

let numberFormatter = NumberFormatter()
numberFormatter.minimumFractionDigits = 1
numberFormatter.groupingSeparator = " "
let num = 222.3333
_ = num.precision(2)

Full sample

func option1<T: Numeric>(value: T, numerFormatter: NumberFormatter? = nil) {
    print("Type: \(type(of: value))")
    print("Original Value: \(value)")
    let value1 = value.precision(2)
    print("value1 = \(value1 != nil ? "\(value1!)" : "nil")")
    let value2 = value.precision(5)
    print("value2 = \(value2 != nil ? "\(value2!)" : "nil")")
    if let value1 = value1, let value2 = value2 {
        print("value1 + value2 = \(value1 + value2)")
    }
    print("")
}

func option2<T: Numeric>(value: T, numberFormatter: NumberFormatter) {
    print("Type: \(type(of: value))")
    print("Original Value: \(value)")
    let value1 = value.precision(with: numberFormatter)
    print("formated value = \(value1 != nil ? "\(value1!)" : "nil")\n")
}

func test(with double: Double) {
    print("===========================\nTest with: \(double)\n")
    let float = Float(double)
    let float32 = Float32(double)
    let float64 = Float64(double)
    let float80 = Float80(double)
    let cgfloat = CGFloat(double)

    // Exapmle 1
    print("-- Option1\n")
    option1(value: double)
    option1(value: float)
    option1(value: float32)
    option1(value: float64)
    option1(value: float80)
    option1(value: cgfloat)

    // Exapmle 2

    let numberFormatter = NumberFormatter()
    numberFormatter.formatterBehavior = .behavior10_4
    numberFormatter.minimumIntegerDigits = 1
    numberFormatter.minimumFractionDigits = 4
    numberFormatter.maximumFractionDigits = 9
    numberFormatter.usesGroupingSeparator = true
    numberFormatter.groupingSeparator = " "
    numberFormatter.groupingSize = 3

    print("-- Option 2\n")
    option2(value: double, numberFormatter: numberFormatter)
    option2(value: float, numberFormatter: numberFormatter)
    option2(value: float32, numberFormatter: numberFormatter)
    option2(value: float64, numberFormatter: numberFormatter)
    option2(value: float80, numberFormatter: numberFormatter)
    option2(value: cgfloat, numberFormatter: numberFormatter)
}

test(with: 123.22)
test(with: 1234567890987654321.0987654321)

Output

===========================
Test with: 123.22

-- Option1

Type: Double
Original Value: 123.22
value1 = 123.22
value2 = 123.22
value1 + value2 = 246.44

Type: Float
Original Value: 123.22
value1 = nil
value2 = nil

Type: Float
Original Value: 123.22
value1 = nil
value2 = nil

Type: Double
Original Value: 123.22
value1 = 123.22
value2 = 123.22
value1 + value2 = 246.44

Type: Float80
Original Value: 123.21999999999999886
value1 = nil
value2 = nil

Type: CGFloat
Original Value: 123.22
value1 = 123.22
value2 = 123.22
value1 + value2 = 246.44

-- Option 2

Type: Double
Original Value: 123.22
formatted value = 123.2200

Type: Float
Original Value: 123.22
formatted value = 123.220001221

Type: Float
Original Value: 123.22
formatted value = 123.220001221

Type: Double
Original Value: 123.22
formatted value = 123.2200

Type: Float80
Original Value: 123.21999999999999886
formatted value = nil

Type: CGFloat
Original Value: 123.22
formatted value = 123.2200

===========================
Test with: 1.2345678909876544e+18

-- Option1

Type: Double
Original Value: 1.2345678909876544e+18
value1 = 1.23456789098765e+18
value2 = 1.23456789098765e+18
value1 + value2 = 2.4691357819753e+18

Type: Float
Original Value: 1.234568e+18
value1 = nil
value2 = nil

Type: Float
Original Value: 1.234568e+18
value1 = nil
value2 = nil

Type: Double
Original Value: 1.2345678909876544e+18
value1 = 1.23456789098765e+18
value2 = 1.23456789098765e+18
value1 + value2 = 2.4691357819753e+18

Type: Float80
Original Value: 1234567890987654400.0
value1 = nil
value2 = nil

Type: CGFloat
Original Value: 1.2345678909876544e+18
value1 = 1.23456789098765e+18
value2 = 1.23456789098765e+18
value1 + value2 = 2.4691357819753e+18

-- Option 2

Type: Double
Original Value: 1.2345678909876544e+18
formatted value = 1 234 567 890 987 650 000.0000

Type: Float
Original Value: 1.234568e+18
formatted value = 1 234 567 939 550 610 000.0000

Type: Float
Original Value: 1.234568e+18
formatted value = 1 234 567 939 550 610 000.0000

Type: Double
Original Value: 1.2345678909876544e+18
formatted value = 1 234 567 890 987 650 000.0000

Type: Float80
Original Value: 1234567890987654400.0
formatted value = nil

Type: CGFloat
Original Value: 1.2345678909876544e+18
formatted value = 1 234 567 890 987 650 000.0000

Answer 4

Plenty of good answers above, but sometimes a pattern is more appropriate than the "%.3f" sort of gobbledygook. Here's my take using a NumberFormatter in Swift 3.

extension Double {
  func format(_ pattern: String) -> String {
    let formatter = NumberFormatter()
    formatter.format = pattern
    return formatter.string(from: NSNumber(value: self))!
  }    
}

let n1 = 0.350, n2 = 0.355
print(n1.format("0.00#")) // 0.35
print(n2.format("0.00#")) // 0.355

Here I wanted 2 decimals to be always shown, but the third only if it wasn't zero.

Answer 5

@Christian Dietrich:

instead of:

var k = 1.0
    for i in 1...right+1 {
        k = 10.0 * k
    }
let n = Double(Int(left*k)) / Double(k)
return "\(n)"

it could also be:

let k = pow(10.0, Double(right))
let n = Double(Int(left*k)) / k
return "\(n)"

[correction:] Sorry for confusion* - Of course this works with Doubles. I think, most practical (if you want digits to be rounded, not cut off) it would be something like that:

infix operator ~> {}
func ~> (left: Double, right: Int) -> Double {
    if right <= 0 {
        return round(left)
    }
    let k = pow(10.0, Double(right))
    return round(left*k) / k
}

For Float only, simply replace Double with Float, pow with powf and round with roundf.
Update: I found that it is most practical to use return type Double instead of String. It works the same for String output, i.e.:

println("Pi is roughly \(3.1415926 ~> 3)")

prints: Pi is roughly 3.142
So you can use it the same way for Strings (you can even still write: println(d ~> 2)), but additionally you can also use it to round values directly, i.e.:

d = Double(slider.value) ~> 2

or whatever you need …

Answer 6

Swift 4 Xcode 10 Update

extension Double {
    var asNumber:String {
        if self >= 0 {
            let formatter = NumberFormatter()
            formatter.numberStyle = .none
            formatter.percentSymbol = ""
            formatter.maximumFractionDigits = 2
            return "\(formatter.string(from: NSNumber(value: self)) ?? "")"
        }
        return ""
    }
}