Unsigned Int to negative Int in Swift

Question

I'm working with the Pebble accelerometer data and need to convert the unsigned representations of negative ints to normal numbers (note the first two for example).

[X: 4294967241, Z: 4294967202, Y: 22] [X: 4294967278, Z: 4294967270, Y: 46] [X: 4294967274, Z: 4294967278, Y: 26] [X: 4294967221, Z: 85, Y: 4294967280] [X: 4294967247, Z: 117, Y: 4294967266]

Using Objective C I've managed to do so by the simple [number intValue]. However, using Swift I can't find a way to do so. I've tried Int(number), but I get the same value.

Am I reading unsigned negative int right? You can also just use abs to get an Int's absolute value. — Fred Faust, Aug 07 '15 at 15:24
An unsigned negative number doesn't exist, do you mean negative signed maybe? — Kametrixom, Aug 07 '15 at 15:27
Can you add more examples of values your getting? Also what's the type of those values? UInt32? Int32? Int16? — Kametrixom, Aug 07 '15 at 15:41
@Kametrixom unsigned representation of a negative number. I've updated the post with more values. — Marti Serra Vivancos, Aug 07 '15 at 15:46
@Kametrixom string, but I was only trying to convert the numbers. Anyway, your code worked :) — Marti Serra Vivancos, Aug 07 '15 at 15:56
Wow, really weird that you only have a string, something must have gone wrong somewhere. Btw if my answer is really working for you, you can accept it ;) — Kametrixom, Aug 07 '15 at 16:01
Compare [Converting signed to unsigned in Swift](http://stackoverflow.com/questions/25666846/converting-signed-to-unsigned-in-swift) (which is essentially the same problem, only into the opposite direction). — Martin R, Aug 07 '15 at 16:06
It would also be interesting to know *how* you retrieve those values. I would imagine that this can be fixed earlier. — Martin R, Aug 07 '15 at 16:10

score 5 · Answer 1 · edited May 23 '17 at 11:54

5

You can always do an unsafeBitcast if you received the wrong type:

unsafeBitCast(UInt32(4294967009), Int32.self)  // -287

This for example converts an UInt32 to an Int32

EDIT: vacawama showed a better solution without the nasty unsafe:

Int32(bitPattern: 4294967009)

Thanks a lot! Use this one instead, it's safe (obviously) and probably faster

edited May 23 '17 at 11:54

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answered Aug 07 '15 at 15:40

Kametrixom

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Can also be done with `Int32(bitPattern: 4294967009)` which doesn't have that nasty `unsafe` word in it. :-) – vacawama Aug 07 '15 at 16:04

vacawama · Answer 2 · 2015-08-07T16:06:50.400

You can use bitPattern in the constructor to convert unsigned to signed:

let x = Int32(bitPattern: 4294967009)
print(x)  // prints "-287"

or if your value is already stored in a variable:

let x: UInt32 = 4294967241
let y = Int32(bitPattern: x)
print(y)  // prints "-55"

If your value is stored as a 64-bit UInt, you need to use truncatingBitPattern when converting to Int32:

let x: UInt = 4294967241
let y = Int(Int32(truncatingBitPattern: x))
print(y)  // prints "-55"

GoatInTheMachine · Answer 3 · 2015-08-07T15:32:57.333

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Are you just trying to convert an unsigned representation of a negative number into it's signed representation? If so, this should work:

let negativeValue = Int((incomingValue - 0xffffffff) % 0xffffffff)

edited Aug 07 '15 at 15:32

answered Aug 07 '15 at 15:25

GoatInTheMachine

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`%` has a higher precendence than `-`, I think you need some brackets – Kametrixom Aug 07 '15 at 15:31
That's not working, it just turns all my values into negative. – Marti Serra Vivancos Aug 07 '15 at 15:33
@MartiSerraVivancos Maybe the values ARE negative?? – Kametrixom Aug 07 '15 at 15:34
@Kametrixom not all of them are. The code is turning positive numbers into negative and unsigned representations of negative into large negative ints. Like this: [X: -4294967009, Z: -4294967262, Y: -112] – Marti Serra Vivancos Aug 07 '15 at 15:37

Unsigned Int to negative Int in Swift

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