What is the replacement for isDigit() for characters in Swift?

With Swift 5, according to your needs, you may choose one of the following ways in order to solve you problem.

#1. Using Character's isNumber property

Character has a property called isNumber. isNumber has the following declaration:

var isNumber: Bool { get }

A Boolean value indicating whether this character represents a number.

The Playground sample codes below show how to check if a character represents a number using isNumber: 

let character: Character = "9"
print(character.isNumber) // true

let character: Character = "½"
print(character.isNumber) // true

let character: Character = "④"
print(character.isNumber) // true

let character: Character = "1⃣"
print(character.isNumber) // true

let character: Character = "1️⃣"
print(character.isNumber) // true

let character: Character = "৯"
print(character.isNumber) // true

let character: Character = "𝟙"
print(character.isNumber) // true

let character: Character = "F"
print(character.isNumber) // false

#2. Using Character's isWholeNumber property

If you want to check if a character represents a whole number, you can use Character's isWholeNumber property:

let character: Character = "9"
print(character.isWholeNumber) // true

let character: Character = "½"
print(character.isWholeNumber) // false

let character: Character = "④"
print(character.isWholeNumber) // true

let character: Character = "1⃣"
print(character.isWholeNumber) // false

let character: Character = "1️⃣"
print(character.isWholeNumber) // false

let character: Character = "৯"
print(character.isWholeNumber) // true

let character: Character = "𝟙"
print(character.isWholeNumber) // true

let character: Character = "F"
print(character.isWholeNumber) // false

#3. Using Unicode.Scalar.Properties's generalCategory property and Unicode.GeneralCategory.decimalNumber

The Playground sample codes below show how to check if the first Unicode scalar of a character is a decimal number using generalCategory and Unicode.GeneralCategory.decimalNumber:

let character: Character = "9"
let scalar = character.unicodeScalars.first! // DIGIT NINE
print(scalar.properties.generalCategory == .decimalNumber) // true

let character: Character = "½"
let scalar = character.unicodeScalars.first! // VULGAR FRACTION ONE HALF
print(scalar.properties.generalCategory == .decimalNumber) // false

let character: Character = "④"
let scalar = character.unicodeScalars.first! // CIRCLED DIGIT FOUR
print(scalar.properties.generalCategory == .decimalNumber) // false

let character: Character = "1⃣"
let scalar = character.unicodeScalars.first! // DIGIT ONE
print(scalar.properties.generalCategory == .decimalNumber) // true

let character: Character = "1️⃣"
let scalar = character.unicodeScalars.first! // DIGIT ONE
print(scalar.properties.generalCategory == .decimalNumber) // true

let character: Character = "৯"
let scalar = character.unicodeScalars.first! // BENGALI DIGIT NINE
print(scalar.properties.generalCategory == .decimalNumber) // true

let character: Character = "𝟙"
let scalar = character.unicodeScalars.first! // MATHEMATICAL DOUBLE-STRUCK DIGIT ONE
print(scalar.properties.generalCategory == .decimalNumber) // true

let character: Character = "F"
let scalar = character.unicodeScalars.first! // LATIN CAPITAL LETTER F
print(scalar.properties.generalCategory == .decimalNumber) // false

#4. Using Unicode.Scalar.Properties's generalCategory property and Unicode.GeneralCategory.otherNumber

Similarly, you can check that the first Unicode scalar of a character corresponds to the category Other_Number in the Unicode Standard using generalCategory and Unicode.GeneralCategory.otherNumber:

let character: Character = "9"
let scalar = character.unicodeScalars.first!
print(scalar.properties.generalCategory == .otherNumber) // false

let character: Character = "½"
let scalar = character.unicodeScalars.first!
print(scalar.properties.generalCategory == .otherNumber) // true

let character: Character = "④"
let scalar = character.unicodeScalars.first!
print(scalar.properties.generalCategory == .otherNumber) // true

let character: Character = "1⃣"
let scalar = character.unicodeScalars.first!
print(scalar.properties.generalCategory == .otherNumber) // false

let character: Character = "1️⃣"
let scalar = character.unicodeScalars.first!
print(scalar.properties.generalCategory == .otherNumber) // false

let character: Character = "৯"
let scalar = character.unicodeScalars.first!
print(scalar.properties.generalCategory == .otherNumber) // false

let character: Character = "𝟙"
let scalar = character.unicodeScalars.first!
print(scalar.properties.generalCategory == .otherNumber) // false

let character: Character = "F"
let scalar = character.unicodeScalars.first!
print(scalar.properties.generalCategory == .otherNumber) // false

#5. Using CharacterSet's decimalDigits property

As an alternative, you can import Foundation and check if CharacterSet.decimalDigits contains the first Unicode scalar of a character:

import Foundation

let character: Character = "9"
let scalar = character.unicodeScalars.first!
print(CharacterSet.decimalDigits.contains(scalar)) // true

import Foundation

let character: Character = "½"
let scalar = character.unicodeScalars.first!
print(CharacterSet.decimalDigits.contains(scalar)) // false

import Foundation

let character: Character = "④"
let scalar = character.unicodeScalars.first!
print(CharacterSet.decimalDigits.contains(scalar)) // false

import Foundation

let character: Character = "1⃣"
let scalar = character.unicodeScalars.first!
print(CharacterSet.decimalDigits.contains(scalar)) // true

import Foundation

let character: Character = "1️⃣"
let scalar = character.unicodeScalars.first!
print(CharacterSet.decimalDigits.contains(scalar)) // true

import Foundation

let character: Character = "৯"
let scalar = character.unicodeScalars.first!
print(CharacterSet.decimalDigits.contains(scalar)) // true

import Foundation

let character: Character = "𝟙"
let scalar = character.unicodeScalars.first!
print(CharacterSet.decimalDigits.contains(scalar)) // true

import Foundation

let character: Character = "F"
let scalar = character.unicodeScalars.first!
print(CharacterSet.decimalDigits.contains(scalar)) // false

#6. Using Unicode.Scalar.Properties's numericType

Apple documentation states for numericType:

For scalars that represent a number, numericType is the numeric type of the scalar. For all other scalars, this property is nil.

The sample codes below show the possible numeric type (decimal, digit or numeric) for the first scalar of a given character:

let character: Character = "9"
let scalar = character.unicodeScalars.first!
print(scalar.properties.numericType) // Optional(Swift.Unicode.NumericType.decimal)

let character: Character = "½"
let scalar = character.unicodeScalars.first!
print(scalar.properties.numericType) // Optional(Swift.Unicode.NumericType.numeric)

let character: Character = "④"
let scalar = character.unicodeScalars.first!
print(scalar.properties.numericType) // Optional(Swift.Unicode.NumericType.digit)

let character: Character = "1⃣"
let scalar = character.unicodeScalars.first!
print(scalar.properties.numericType) // Optional(Swift.Unicode.NumericType.decimal)

let character: Character = "1️⃣"
let scalar = character.unicodeScalars.first!
print(scalar.properties.numericType) // Optional(Swift.Unicode.NumericType.decimal)

let character: Character = "৯"
let scalar = character.unicodeScalars.first!
print(scalar.properties.numericType) // Optional(Swift.Unicode.NumericType.decimal)

let character: Character = "𝟙"
let scalar = character.unicodeScalars.first!
print(scalar.properties.numericType) // Optional(Swift.Unicode.NumericType.decimal)

let character: Character = "F"
let scalar = character.unicodeScalars.first!
print(scalar.properties.numericType) // nil

The "problem" is that a Swift character does not directly correspond to a Unicode code point, but represents an "extended grapheme cluster" which can consist of multiple Unicode scalars. For example

let c : Character = "🇺🇸" // REGIONAL INDICATOR SYMBOL LETTERS US

is actually a sequence of two Unicode scalars.

If we ignore this fact then you can retrieve the initial Unicode scalar of the character (compare How can I get the Unicode code point(s) of a Character?) and test its membership in a character set:

let c : Character = "5"

let s = String(c).unicodeScalars
let uni = s[s.startIndex]

let digits = NSCharacterSet.decimalDigitCharacterSet()
let isADigit = digits.longCharacterIsMember(uni.value)

This returns "true" for the characters "0" ... "9", but actually for all Unicode scalars of the "decimal digit category", for example:

let c1 : Character = "৯" // BENGALI DIGIT NINE U+09EF
let c2 : Character = "𝟙" // MATHEMATICAL DOUBLE-STRUCK DIGIT ONE U+1D7D9

If you care only for the (ASCII) digits "0" ... "9", then the easiest method is probably:

if c >= "0" && c <= "9" { }

or, using ranges:

if "0"..."9" ~= c { }

Update: As of Swift 5 you can check for ASCII digits with

if c.isASCII && c.isNumber { }

using the “Character properties“ introduced with SE-0221.

This solves also the problem with digits modified by a variation selected U+FE0F, like the Keycap Emoji "1️⃣". (Thanks to Lukas Kukacka for reporting this problem.)

let c: Character = "1️⃣"
print(Array(c.unicodeScalars)) // ["1", "\u{FE0F}", "\u{20E3}"]
print(c.isASCII && c.isNumber) // false

Tags:

Swift

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