Include a UTF8 character literal in a [UInt8] array or Data - swift

Question

I would like something similar to:
let a = ["v".utf8[0], 1, 2]
The closest I have figured out is:
let a = [0x76, 1, 2]
and
"v".data(using: String.Encoding.utf8)! + [1, 2]
Note: Either [UInt8] or Data is an acceptable type.

Answer 1

String's UTF8View is not indexed by an Int, rather it's own String.UTF8View.Index type, therefore in order to include the first byte of a UTF-8 sequence of a given string in your array literal, you could use its first property instead:
let a = ["v".utf8.first!, 1, 2] // [118, 1, 2]
If there's more than one byte in the sequence, you can concatenate the UTF-8 bytes with an array literal simply by using the + operator:
let a = "😀".utf8 + [1, 2] // [240, 159, 152, 128, 1, 2]
Also note that your example to concatenate a [UInt8] to a Data could be shortened slightly to:
let a = "v".data(using: .utf8)! + [1, 2] // Data with bytes [0x76, 0x1, 0x2]

Answer 2

There is a specific UInt8 initializer (introduced in Swift 2.2+):
let a = [UInt8(ascii:"v"), 1 ,2]

Answer 3

(Some addendums to the already posted answers; regarding UnicodeScalar's in particular)
In you question you've used a literal "v" as the base instance to be converted to UInt8; we don't really know if this is a String or e.g. UnicodeScalar in your actual use case. The accepted answer shows some neat approaches in case you are working wit a String instance.
In case you happen to be working with a UnicodeScalar instance (rather than a String), one answer has already mentioned the init(ascii:) initializer of UInt8. You should take care however, to verify that the UnicodeScalar instance used in this initializer is indeed one that that fits within ASCII character encoding; the majority of UnicodeScalar values will not (which will lead to a runtime exeception for this initializer). You may use e.g. the isASCII property of UnicodeScalar to verify this fact prior to making use of the initializer.
let ucScalar: UnicodeScalar = "z"
var a = [UInt8]()
if ucScalar.isASCII {
a = [UInt8(ascii: ucScalar), 1, 2]
}
else {
// ... unexpected but not a runtime error
}
Another approach, in case you'd like to encode the full UnicodeScalar into UInt8 format (even for UnicodeScalar's that cannot be single-byte ASCII endoded) is using the encode(_:into:) method of UTF8:
let ucScalar: UnicodeScalar = "z"
var bytes: [UTF8.CodeUnit] = []
UTF8.encode(ucScalar, into: { bytes.append($0) })
bytes += [1, 2]
print(bytes) // [122, 1, 2]
// ...
let ucScalar: UnicodeScalar = "\u{03A3}" // Σ
var bytes: [UTF8.CodeUnit] = []
UTF8.encode(ucScalar, into: { bytes.append($0) })
bytes += [1, 2]
print(bytes) // [206, 163, 1, 2]