How to convert bytes to a float value in swift?

问题:

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

回答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 } ))
}

回答2:

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