如何在 swift 中将字节转换为浮点值？

Question

This is my code to convert byte data to float. I tried every answers given in this site. I am getting exponential value for this "<44fa0000>" byte data

    static func returnFloatValue(mutableData:NSMutableData)->Float
    {
       let qtyRange = mutableData.subdataWithRange(NSMakeRange(0, 4))
       let qtyString = String(qtyRange)
       let qtyTrimString = qtyString.stringByTrimmingCharactersInSet(NSCharacterSet(charactersInString: "<>"))
       let qtyValue =  Float(strtoul(qtyTrimString, nil, 16)/10)
       return qtyValue
    }

Thanks

Answer 1

<44fa0000> is the big-endian memory representation of the binary floating point number 2000.0 . To get the number back from the data, you have to read it into an UInt32 first, convert from big-endian to host byteorder, and then cast the result to a Float .

In Swift 2 that would be

func floatValueFromData(data: NSData) -> Float {
    return unsafeBitCast(UInt32(bigEndian: UnsafePointer(data.bytes).memory), Float.self)
}

Example:

let bytes: [UInt8] =  [0x44, 0xFA, 0x00, 0x00]
let data = NSData(bytes: bytes, length: 4)

print(data) // <44fa0000>
let f = floatValueFromData(data)
print(f) // 2000.0

In Swift 3 you would use Data instead of NSData , and the unsafeBitCast can be replaced by the Float(bitPattern:) initializer:

func floatValue(data: Data) -> Float {
    return Float(bitPattern: UInt32(bigEndian: data.withUnsafeBytes { $0.pointee } ))
}

In Swift 5 the withUnsafeBytes() method of Data calls the closure with an (untyped) UnsafeRawBufferPointer , and you can load() the value from the raw memory:

func floatValue(data: Data) -> Float {
    return Float(bitPattern: UInt32(bigEndian: data.withUnsafeBytes { $0.load(as: UInt32.self) }))
}

Answer 2

Here is some swift 5 :

let data = Data([0x44, 0xfa, 0x00, 0x00]) // 0x44fa0000
let floatNb:Float = data.withUnsafeBytes { $0.load(as: Float.self) }

// note that depending on the input endianess, you could add .reversed() to data
let floatNb:Float = data.reversed().withUnsafeBytes { $0.load(as: Float.self) }

WARNING : this sample throws if your Data is under 4 bytes..

.

Safe Data extension:

extension Data {
    enum Endianess {
        case little
        case big
    }

    func toFloat(endianess: Endianess = .little) -> Float? {
        guard self.count <= 4 else { return nil }

        switch endianess {
        case .big:
            let data = [UInt8](repeating: 0x00, count: 4-self.count) + self
            return data.withUnsafeBytes { $0.load(as: Float.self) }
        case .little:
            let data = self + [UInt8](repeating: 0x00, count: 4-self.count)
            return data.reversed().withUnsafeBytes { $0.load(as: Float.self) }
        }
    }
}

Tests:

let opData        = Data([0x44, 0xFA, 0x00, 0x00])
let nb42          = Data([0x42, 0x28])
let nb42bigEndian = Data([0x28, 0x42])
let tooBig        = Data([0x44, 0xFA, 0x00, 0x00, 0x00])

print("opData:        \(opData.toFloat())")
print("nb42:          \(nb42.toFloat())")
print("nb42bigEndian: \(nb42bigEndian.toFloat(endianess: .big))")
print("tooBig:        \(tooBig.toFloat())")

you may find a faster way but this was good enough for my needs

Answer 3

Use this function:

static func returnFloatValue(data: NSMutableData) -> Float {
    let bytes = [UInt8](data as Data)
    var f: Float = 0

    memcpy(&f, bytes, 4)
    return f
}

And you can see it in action here:

var initialValue: Float = 19.200

let data = NSMutableData(bytes: &initialValue, length: 4)

func returnFloatValue(data: NSMutableData) -> Float {
    let bytes = [UInt8](data as Data)
    var f: Float = 0

    memcpy(&f, bytes, 4)
    return f
}

var result:Float = returnFloatValue(data: data)

print("f=\(result)")// f=19.2

Answer 4

For 64 bit values the code is:

static func longBitsToDouble(x : Int64) -> Float64
{ return Float64(bitPattern: UInt64(x)) } 

Swift 4+.