如何在swift中将字节转换为浮点值？

Question

这是我将字节数据转换为float的代码.我尝试了这个网站给出的每个答案.我得到了这个"<44fa0000>"字节数据的指数值

    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
    }

谢谢

Answer 1

<44fa0000>是二进制浮点数的big-endian内存表示2000.0.要从数据中获取数字,您必须将其读入UInt32第一个,从big-endian转换为host byteorder,然后将结果转换为a Float.

在Swift 2中就是这样

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

例:

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

在Swift 3中,您将使用Data而不是NSData,并且 unsafeBitCast 可以由Float(bitPattern:) 初始化程序替换:

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

Answer 2

这是一些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) }

警告：如果您的数据小于 4 字节，则此示例会抛出。

.

安全数据扩展：

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) }
        }
    }
}

测试：

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())")

您可能会找到更快的方法，但这足以满足我的需求