Guide to Swift Numbers Sample Code

This repository contains sample code used in the
Flight School Guide to Swift Numbers.

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Chapter 1

Chapter 1 is a conceptual deep-dive into
how numbers work on computers in general
and in Swift specifically.

Floating Point Approximates

You know how floating-point arithmetic
sometimes produces unexpected results,
like 0.1 + 0.2 != 0.3?
(If not, go ahead and read the
first chapter for free!)

This sample code implements an “approximately equals” operator (==~)
for floating-point number types.

0.1 + 0.2 == 0.3 // false
0.1 + 0.2 ==~ 0.3 // true

(0.1 + 0.2).isApproximatelyEqual(to: 0.3, within: .ulpOfOne) // true

Floating Point Environment

Normally, you can’t tell the difference between nan and signalingNaN.
That’s because Swift doesn’t expose the floating-point environment
in its standard library.

We can still access it from Darwin, though.
And that’s what this playground demonstrates:

do {
    try detectingFloatingPointErrors(flags: .invalid) {
        Double.signalingNaN + 1
    }
} catch {
    print("Error: \(error)")
}

Chapter 2

Chapter 2 is all about number formatting.
The sample code in this chapter offers a comprehensive survey
of the various formatting styles of NumberFormatter,
and how they work in different locales.

Cardinal Numbers

let formatter = NumberFormatter()
formatter.string(for: 4) // 4

formatter.numberStyle = .spellOut
formatter.string(for: 4) // four

Ordinal Numbers

let formatter = NumberFormatter()
formatter.numberStyle = .ordinal
formatter.string(for: 1) // 1st

Decimal Numbers

let formatter = NumberFormatter()
formatter.numberStyle = .decimal
formatter.string(for: 1234567.89) 1,234,567.89

Significant Digits

let formatter = NumberFormatter()
formatter.usesSignificantDigits = true

formatter.maximumSignificantDigits = 2

formatter.string(from: 123) // 120
formatter.string(from: 123456) // 120000
formatter.string(from: 123.456) // 120
formatter.string(from: 1.230000) // 1.2
formatter.string(from: 0.00123) // 0.0012

Integer and Fraction Digits

let formatter = NumberFormatter()
formatter.usesSignificantDigits = false // default

formatter.minimumIntegerDigits = 4
formatter.minimumFractionDigits = 2

formatter.string(from: 123) // 0123.00
formatter.string(from: 123456) // 123456.00
formatter.string(from: 123.456) // 0123.46
formatter.string(from: 1.230000) // 0001.23
formatter.string(from: 0.00123) // 0000.00

Rounding Modes

let formatter = NumberFormatter()
formatter.numberStyle = .decimal
formatter.maximumFractionDigits = 1

let numbers = [1.2, 1.22, 1.25, 1.27, -1.25]
let modes: [NumberFormatter.RoundingMode] = [.ceiling, .floor, .up, .down, .halfUp, .halfDown, .halfEven]

for mode in modes {
    formatter.roundingMode = mode

    for number in numbers {
        formatter.string(for: number)
    }
}

Scientific Notation

let formatter = NumberFormatter()
formatter.numberStyle = .scientific
formatter.string(for: 12345.6789) // 1.23456789E4

Percentages

let formatter = NumberFormatter()
formatter.numberStyle = .percent
formatter.string(for: 0.12) // 12%

Currencies

let formatter = NumberFormatter()

let identifiers =
    ["en-US", "en-GB", "de-DE", "ja-JP"]
let styles: [NumberFormatter.Style] =
    [.currency, .currencyAccounting, .currencyISOCode, .currencyPlural]

for style in styles {
    formatter.numberStyle = style
    for identifier in identifiers {
        formatter.locale = Locale(identifier: identifier)
        formatter.string(for: 1234.567)
    }
}

Custom Formats

let formatter = NumberFormatter()
formatter.numberStyle = .decimal

// Format with thousands and decimal separator
// that rounds to the nearest five tenths
formatter.format = "#,##0.5"

formatter.locale = Locale(identifier: "en-US")
formatter.string(for: 1234.567) // 1,234.5

formatter.locale = Locale(identifier: "fr-FR")
formatter.string(for: 1234.567) // 1 234,5

Chapter 3

Chapter 3 shows the correct way to represent and work with money in code.

Money

let prices: [Money<USD>] = [2.19, 5.39, 20.99, 2.99, 1.99, 1.99, 0.99]
let subtotal = prices.reduce(0.00, +)
let tax = 0.08 * subtotal
let total = subtotal + tax // $39.45

Currency Converter

let EURtoUSD = CurrencyPair<EUR, USD>(rate: 1.17) // as of June 1st, 2018

let euroAmount: Money<EUR> = 123.45
let dollarAmount = EURtoUSD.convert(euroAmount) // $144.44

Chapter 4

Chapter 4 covers Foundation’s units and measurements APIs.

Natural Scale

let lengthOfRoom = Measurement<UnitLength>(value: 8, unit: .meters)
let distanceToAirport = Measurement<UnitLength>(value: 16, unit: .kilometers)

let formatter = MeasurementFormatter()
formatter.unitOptions = .naturalScale
formatter.string(from: lengthOfRoom) // 26.247 ft
formatter.string(from: distanceToAirport) // 9.942 mi

Provided Units

let ingotMass = Measurement<UnitMass>(value: 400, unit: .ouncesTroy)

let formatter = MeasurementFormatter()
formatter.unitOptions = .providedUnit
formatter.string(from: ingotMass) // 400 oz t

Configuring Precision

let barometerReading = Measurement<UnitPressure>(value: 29.9, unit: .inchesOfMercury)
let pressureInMillibars = barometerReading.converted(to: .millibars)

let formatter = MeasurementFormatter()
formatter.unitOptions = .providedUnit
formatter.numberFormatter.usesSignificantDigits = true
formatter.numberFormatter.maximumSignificantDigits = 3
formatter.string(from: pressureInMillibars) // 1,010 mbar

Temperature Formatting

let temperatureInF = Measurement<UnitTemperature>(value: 72, unit: .fahrenheit)
let temperatureInC = Measurement<UnitTemperature>(value: 20.5, unit: .celsius)

let formatter = MeasurementFormatter()
formatter.locale = Locale(identifier: "en-US")
formatter.string(from: temperatureInF) // 72°F
formatter.string(from: temperatureInC) // 68.9°F

formatter.locale = Locale(identifier: "fr-FR")
formatter.string(from: temperatureInF) // 22,222 °C
formatter.string(from: temperatureInC) // 20,5 °C

formatter.unitOptions = .temperatureWithoutUnit

formatter.locale = Locale(identifier: "en-US")
formatter.string(from: temperatureInF) // 72°
formatter.string(from: temperatureInC) // 20.5° (!)

formatter.locale = Locale(identifier: "fr-FR")
formatter.string(from: temperatureInF) // 72° (!)
formatter.string(from: temperatureInC) // 20,5°

Units Interoperability

let loggedFlyingTime =
    Measurement<UnitDuration>(value: 220, unit: .hours)

let formatter = DateComponentsFormatter()
formatter.allowedUnits = [.day]
formatter.unitsStyle = .full
formatter.includesApproximationPhrase = true
formatter.string(from: loggedFlyingTime) // About 9 days

Chapter 5

Chapter 5 extends what we learned about units in the previous chapter
to transform Xcode Playgrounds into an interactive physical calculator.

Agricultural Flight Planner

let takeoffWeight = (
    emptyPlaneWeight +
    payloadWeight +
    fuelWeight +
    pilotWeight
).converted(to: .pounds)

let canTakeOff = takeoffWeight < maximumTakeoffWeight // ???

License

MIT

About Flight School

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that explores essential topics in iOS and macOS development
through concise, focused guides.

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