Convert a two byte UInt8 array to a UInt16 in Swift

I don't know the syntax for swift, but what about something like:

let a:UInt16 = UInt16(bytes[0]) * 256 + UInt16(bytes[1])

How about

let bytes:[UInt8] = [0x01, 0x02]
let result = (UInt16(bytes[1]) << 8) + UInt16(bytes[0])

With a loop, this easily generalizes to larger byte arrays, and it can be wrapped in a function for readability:

let bytes:[UInt8] = [0x01, 0x02, 0x03, 0x04]

func bytesToUInt(byteArray: [UInt8]) -> UInt {
  assert(byteArray.count <= 4)
  var result: UInt = 0
  for idx in 0..<(byteArray.count) {
    let shiftAmount = UInt((byteArray.count) - idx - 1) * 8
    result += UInt(byteArray[idx]) << shiftAmount
  }
  return result
}

println(bytesToUInt(bytes))    // result is 16909060

In Swift 5 or later you can convert the bytes [UInt8] to UInt16 value using withUnsafeBytes { $0.load(as: UInt16.self) }

let bytes: [UInt8] = [1, 2]

loading as UInt16

let uint16 = bytes.withUnsafeBytes { $0.load(as: UInt16.self) }    // 513 

---

To get rid of the verbosity we can create a generic method extending ContiguousBytes:

extension ContiguousBytes {
    func object<T>() -> T {
        withUnsafeBytes { $0.load(as: T.self) }
    }
}

---

Usage:

let bytes: [UInt8] = [1, 2]
let uint16: UInt16 = bytes.object()    // 513

---

And to access the bytes anywhere in the collection:

extension Data {
    func subdata<R: RangeExpression>(in range: R) -> Self where R.Bound == Index {
        subdata(in: range.relative(to: self) )
    }
    func object<T>(at offset: Int) -> T {
        subdata(in: offset...).object()
    }
}

---

extension Sequence where Element == UInt8  {
    var data: Data { .init(self) }
}

---

extension Collection where Element == UInt8, Index == Int {
    func object<T>(at offset: Int = 0) -> T {
        data.object(at: offset)
    }
}

---

Usage:

let bytes: [UInt8] = [255, 255, 1, 2]
let uintMax: UInt16 = bytes.object()      // 65535 at offset zero
let uint16: UInt16 = bytes.object(at: 2)  // 513   at offset two

If you want to go via NSData then it would work like this:

let bytes:[UInt8] = [0x01, 0x02]
println("bytes: \(bytes)") // bytes: [1, 2]
let data = NSData(bytes: bytes, length: 2)
print("data: \(data)") // data: <0102>

var u16 : UInt16 = 0 ; data.getBytes(&u16)
// Or:
let u16 = UnsafePointer<UInt16>(data.bytes).memory

println("u16: \(u16)") // u16: 513

Alternatively:

let bytes:[UInt8] = [0x01, 0x02]
let u16 = UnsafePointer<UInt16>(bytes).memory
print("u16: \(u16)") // u16: 513

Both variants assume that the bytes are in the host byte order.

Update for Swift 3 (Xcode 8):

let bytes: [UInt8] = [0x01, 0x02]
let u16 = UnsafePointer(bytes).withMemoryRebound(to: UInt16.self, capacity: 1) {
    $0.pointee
}
print("u16: \(u16)") // u16: 513