working with bytes (aka UInt8 aka Data)

Question

Created Oct ’18

Replies 3

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Participants 3

Hello,

It may sound like a very trivial question (well I'm a noob after all) but I've been searching everywhere all evening and my head hurts : How do you add bytes ? I want to calculate the sum of 8 bytes:

I tried:

Let data : [UInt8] = [0xFF,0xF0,0x0F,0x11,0xFF,0xFF,0xF0,0x0F]


let total = data.reduce(0, +)
let numberSum = data.reduce(0, { x, y in x + y})


var sum : UInt8 = 0
for i in 0...data.count-1 {
  sum += data[i]
}

reduce() does not seem to work with an array of UInt8 but it works with arrays of Int

The loop does not work either, I cannot find the right format for sum and I cannot manage the overflow when I +=

The final goal of all this is to calculate a simple checksum. I came across some very complex methods and I am sure there is a simplier way. The code below shows how I did it in C++ (Arduino):

byte data[]={0xFF,0xF0,0x0F,0x11,0xFF,0xFF,0xF0,0x0F}; 

byte sum = 0;
  for ( int i=0; i<sizeof(data); i++) {
    sum += data[i];
  }
byte checksum = lowByte(sum);

// sum = 1292 (0x50C) checksum = C

Any idea ?

Thanks

Boost

Answer 1

QuinceyMorris OP

Oct ’18

Accepted Answer

You can't add those things up as UInt8 values, because they overflow — immediately after the first one, which is 256. Instead, you need to do a calculation where you convert each byte to a larger type (Int or UInt or something like that).

It works in C++ (or C) because integers smaller than "int" are automatically promoted to "int" for such calculations. There is no such automatic promotion in Swift.

0

Answer 2

Matt_74 OP

Oct ’18

Thanks QuinceyMorris,

I've already tried that (didn't post all things I tried...) :

var sum : Int = 0
for i in 0...data.count-1 {
  sum += Int(data[i])
}

print(sum)
let hexsum = String(sum, radix: 16)
print(hexsum)

the "sum" is ok but then I don't know how to get the last 8 bits (i.e. lowByte() ). My "hesum" looks ok but is in reality just a useless string...

That was another dead end for me, that's why I didn't post it...

Just now I tried :

let chkSum1 = UInt8(truncatingIfNeeded: sum)
let chkSum2 = sum & 255

and it works !!!

Thanks QM

PS: which of the two methods would be preffered : truncatingIfNeeded or &255 ?

0

Answer 3

DTS Engineer OP

Apple

Oct ’18

which of the two methods would be preffered : truncatingIfNeeded or &255 ?

Neither. If you’re doing a checksum calculation then you want to work in the sum space (

UInt8

in this case) and use wrapping addition, which Swift supports via the

&+

operator. For example:

let data: [UInt8] = [0xFF,0xF0,0x0F,0x11,0xFF,0xFF,0xF0,0x0F]
let sum = data.reduce(0) { (soFar, byte) in
    soFar &+ byte
}

Or if you’re suspicious of all this new-fangled functional programming stuff (-: and just want to use a loop:

let data: [UInt8] = [0xFF,0xF0,0x0F,0x11,0xFF,0xFF,0xF0,0x0F]
var sum = 0 as UInt8
for byte in data {
    sum &+= byte
}

Note that I’m using

&+=

here, which is the wrapping form of

+=

.

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