Is there any format description compatible with Java and Swift?

Question

In Java there is DecimalFormat which supports

• ###.## -> 3.14
• 0 -> 3
• 00.000 -> 03.142
• #.##% -> 314.16%
• pie is ###.## -> pie is 3.14

but I cannot find an equivalent function in Swift for iOS.

There is NumberFormatter, but that does not support pie is ###.##, and setting all properties in code is inconvenient:

formatter.maximumFractionDigits = 2
formatter.numberStyle = .currencyAccounting

I am curious about if there is a format both supported by Java & Swift, that will be very useful in React Native (define the format in js)

Answer 1

(NS)NumberFormatter has positiveFormat and negativeFormat properties, which are format patterns according to the Unicode Technical Standard #35. These seem to be compatible with the Java DecimalFormat.

Examples:

let posNumber = NSNumber(value: Double.pi)
let negNumber = NSNumber(value: -Double.pi)

let f1 = NumberFormatter()
f1.positiveFormat = "00.000"
print(f1.string(from: posNumber)!) // 03.142
print(f1.string(from: negNumber)!) // -03.142

let f2 = NumberFormatter()
f2.positiveFormat = "pie is ###.## "
print(f2.string(from: posNumber)!) // pie is 3.14

The numbers are formatted according to the current locale (so the output can be 3,14 as well). If that is not intended, add

f2.locale = Locale(identifier: "en_US_POSIX")

If you don't set negativeFormat then the positive format with a prepended minus sign will be used for negative numbers. That works well in the first example, but not with custom text:

print(f2.string(from: negNumber)!) // -pie is 3.14

This is solved by setting both the positive and negative format:

let f3 = NumberFormatter()
f3.positiveFormat = "Result is 00.000"
f3.negativeFormat = "Result is -00.000"
print(f3.string(from: posNumber)!) // Result is 03.142
print(f3.string(from: negNumber)!) // Result is -03.142

On macOS, the format property can be used instead, positive and (optional) negative format are separated by a semicolon. In the above examples that would be:

f2.format = "pie is ###.##"

f3.format = "Result is 00.000;Result is -00.000"