How to decrement a Character that contains a digit? [closed]

问题

c is a Character variable in Swift that I happen to know contains a digit from 1 to 9. I want to decrement the value of the digit stored in c, and this is the best I could come up with:

c = Character(String(Int(String(c))! - 1))

Which seems insanely convoluted. Is there a better way?

回答1:

This sounds like a perfect use for UnicodeScalar:

let scalar = UnicodeScalar("8")
let char = Character(UnicodeScalar(scalar.value-1)) // "7"

Or, more verbosely from a character variable:

let c = Character("8")
let str = String(c)
if let scalar = str.unicodeScalars.first {
    let char = Character(UnicodeScalar(scalar.value-1)) // "7"
}

This is pretty messy, as we have to convert the Character into a String, then a UnicodeScalar, and then back to a Character. I'll keep working for towards a better solution.

I've figured out why the UnicodeScalar constructor works with a String literal.

There is a private initializer for _NSSimpleObjCType (which conforms to UnicodeScalar) which takes a the first character of a String and passes it to the rawValue constructor.

NSObjCRuntime.swift

extension _NSSimpleObjCType {
    init?(_ v: UInt8) {
        self.init(rawValue: UnicodeScalar(v))
    }

    init?(_ v: String?) {
        if let rawValue = v?.unicodeScalars.first {
            self.init(rawValue: rawValue)
        } else {
            return nil
        }
    }
}

回答2:

You can create an extension as follow:

extension Character {
    var integerValue: Int {
        return Int(String(self)) ?? 0
    }
    var predecessor: Character {
        return integerValue.predecessor() < 0  ? "0" : String(integerValue.predecessor()).characters.first!
    }
}

var c: Character = "9"
c = c.predecessor  
print(c)  // "8"

来源：https://stackoverflow.com/questions/35489690/how-to-decrement-a-character-that-contains-a-digit

标签

swift

character