Convert from subarray of a bit array to int and has incorrect result

Question

I have found the issue. In order to make question focus on the issue, I edited and reformat the question.

History: Previously I was using NSData instead of Data. I should use Data so I changed the question. Using Data does not solve the problem. I reformat it a little bit to make the problem stands out. See answer if you have the same issue

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I am using Swift 3.1

I want to convert the sub-array of a byte array into Int.

This is what I do here.

// I am extracting every 4 from the dataBytes and convert it to int
  var xDataArray = dataBytes[8...11]
  let xData = Data(buffer: UnsafeBufferPointer(start: &xDataArray, count: xDataArray.count))
  var xVal: Int32 = xData.withUnsafeBytes { $0.pointee }
  xVal = Int32(bigEndian: xVal)

  var yDataArray = dataBytes[12...15]
  let yData = Data(buffer: UnsafeBufferPointer(start: &yDataArray, count: yDataArray.count))
  var yVal: Int32 = yData.withUnsafeBytes { $0.pointee }
  yVal = Int32(bigEndian: yVal)

The xVal and yVal are the same and they seem to be very large random number.

The reason I choose NSData instead of Data, is because if I call data.getBytes(&num, length: MemoryLayout<Int>.size) using Data then it will give a compiler error and force me to pass in Uint8 instead of Int. — nuynait
– nuynait, Commented May 5, 2017 at 16:10
The Data equivalent is very similar: Data(bytes: &src, count: MemoryLayout<Int>.size) — vadian
– vadian, Commented May 5, 2017 at 16:17
I found out that the problem is when I create the sub-array from the dataBytes @vadian I know the constructor for creating the data, however, what I want to do is to convert data into int. When I use Data instead of NSData I need to use copyBytes but it only accepts utf8 as input. Thanks for the help. — nuynait
– nuynait, Commented May 5, 2017 at 16:36
You don't need copyBytes. Data has appropriate API and is much more versatile than NSData. Did you read the duplicate question round trip Swift number types to/from Data and its answers in your linked answer? It's very informative. — vadian
– vadian, Commented May 5, 2017 at 16:41

Community · Accepted Answer · 2020-06-20 09:12:55Z

1

I solved the issue.

I am using dataBytes[8...11] to get the subarray and this is the place where it get me the same result of the NSData.

Instead I use xDataArray:[UInt8] = [dataBytes[8], dataBytes[9], dataBytes[10], dataBytes[11]]

  var xDataArray: [UInt8] = [dataBytes[8], dataBytes[9], dataBytes[10], dataBytes[11]]
  let xData = NSData(bytes: &xDataArray, length: xDataArray.count)
  var xVal: Int32 = 0
  xData.getBytes(&xVal, length: MemoryLayout<Int32>.size)
  xVal = Int32(bigEndian: xVal)

  var yDataArray: [UInt8] = [dataBytes[12], dataBytes[13], dataBytes[14], dataBytes[15]]
  let yData = NSData(bytes: &yDataArray, length: yDataArray.count)
  var yVal: Int32 = 0
  yData.getBytes(&yVal, length: MemoryLayout<Int32>.size)
  yVal = Int32(bigEndian: yVal)

NOTE: If the Int gets from the Server is 32 Bit, Use Int32 instead of Int

edited Jun 20, 2020 at 9:12

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answered May 5, 2017 at 16:33

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