Swift4 Playgrounds Caesar Cipher error

Question

I'm trying to make a caesar cipher in Swift Playgrounds but whenever the letter is for example "W" and I'm trying to shift it by 4, instead of getting "A" I just get an error. If the ascii code + shift doesn't exceed the ascii code for Z, it works just fine, otherwise I get

error: Execution was interrupted, reason: EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0).

Here is my code:

func cipher(messageToCipher: String, shift: UInt32) {
    var ciphredMessage = ""

    for char in messageToCipher.unicodeScalars {
        var unicode : UInt32 = char.value

        if char.value > 64 && char.value < 123 {
            var modifiedShift = shift
            if char.value >= 65 && char.value <= 90 {
                while char.value + modifiedShift > 90 {
                 //return to A
                    modifiedShift -= 26
                }
            } else if char.value >= 97 && char.value <= 122 {
                while char.value + modifiedShift > 122 {
                  //return to a
                    modifiedShift -= 26
                }
            }

            unicode = char.value + modifiedShift
        }

        ciphredMessage += String(UnicodeScalar(unicode)!)
    }

    print(ciphredMessage)
}

Can anyone tell me why I get error when the ascii code for the letter + shift exceeds the "z"'s ascii code?

Answer 1

The shift is UInt32 . So, with var modifiedShift = shift , modifiedShift is inferred to be UInt32 , too. So when you set modifiedShift to 4, and then try to subtract 26 from it, that's not an acceptable UInt32 value.

Bottom line, use signed integers.

Answer 2

The problem is that the value of modifiedShift can be negative which is NOT allowed for a value of type UInt32 so I would suggest to use only Int whenever you can:

// use `Int` for `shift`
func cipher(messageToCipher: String, shift: Int) {
    var ciphredMessage = ""

    for char in messageToCipher.unicodeScalars {
        // convert to `Int`
        var unicode = Int(char.value)

        if unicode > 64 && unicode < 123 {
            var modifiedShift = shift
            if unicode >= 65 && unicode <= 90 {
                while unicode + modifiedShift > 90 {
                    //return to A
                    modifiedShift -= 26
                }
            } else if unicode >= 97 && unicode <= 122 {
                while unicode + modifiedShift > 122 {
                    //return to a
                    modifiedShift -= 26
                }
            }

            unicode += modifiedShift
        }

        ciphredMessage += String(UnicodeScalar(unicode)!)
    }

    print(ciphredMessage)
}

Note: you can also use if case for range matching. The following lines are semantically equivalent:

if unicode > 64 && unicode < 123 { ... }
if case 65..<123 = unicode { ... }
if (65..<123).contains(unicode) { ... }