How to convert an ContiguousArray of Floats into a byte array in Swift?

Question

I'm trying to convert a ContiguousArray of Floats into a byte array in Swift but cannot find a smart way without using a for loop. I have been following previous posts such as How to convert a double into a byte array in swift?, but I am unable to retrieve the contents of the array without iteratively getting each element one by one.

Is there an elegant way to directly "copy" the bytes of a ContiguousArray directly to byte array?

PS: I have successfully implemented converting structures into byte arrays, but not able to do the same with arrays.

Can you please edit your post and show the desired input an output — Leo Dabus, Nov 21 '19 at 18:29

Martin R · Accepted Answer · 2019-11-21T19:42:43.323

You can use the withUnsafeBytes() method to get a buffer pointer to the underlying bytes of the array's contiguous storage, and directly initialize an [UInt8] array from that buffer pointer. Example:

let floatArray: [Float] = [1.0, 2.0]
// Works also with a ContiguousArray:
// let floatArray: ContiguousArray<Float> = [1.0, 2.0]

let byteArray = floatArray.withUnsafeBytes { Array($0) }
print(byteArray) // [0, 0, 128, 63, 0, 0, 0, 64]

Equivalently (based on Leo's suggestion):

let byteArray = floatArray.withUnsafeBytes(Array.init)

The byte array contains the binary representation of the floating point numbers in host byte order (which is little-endian on all current Apple platforms). A conversion to big-endian is possible, but not without an intermediate copy to an integer array:

let floatArray: ContiguousArray<Float> = [1.0, 2.0]
let intArray = floatArray.map { $0.bitPattern.bigEndian }
let byteArray = intArray.withUnsafeBytes(Array.init)
print(byteArray) // 63, 128, 0, 0, 64, 0, 0, 0]

Reverse conversion: A simple method would be

let floatArray2 = byteArray.withUnsafeBytes { Array($0.bindMemory(to: Float.self)) }
print(floatArray2) // [1.0, 2.0]

However, that requires that the element storage of the byte array is properly aligned for floating point numbers. If that is not guaranteed then you can do

var floatArray2 = [Float](repeating: 0.0, count: byteArray.count / MemoryLayout<Float>.stride)
_ = floatArray2.withUnsafeMutableBytes { byteArray.copyBytes(to: $0) }
print(floatArray2) // [1.0, 2.0]

@LeoDabus: Good suggestion! Even `withUnsafeBytes(Array.init)` works (with automatic type inference). — Martin R, Nov 21 '19 at 18:46
Super thanks! The ```Array($0)``` or ```Array.init``` did the trick. Thank you both MartinR @LeoDabus — jiko, Nov 21 '19 at 19:03
@jiko https://meta.stackexchange.com/questions/5234/how-does-accepting-an-answer-work — Leo Dabus, Nov 22 '19 at 16:14

How to convert an ContiguousArray of Floats into a byte array in Swift?

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