Converting bytes to floats in Swift when receiving bluetooth communications [duplicate] - ios

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+.