How to convert bytes to a float value in swift?

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

`1.15723e+09` (what you call "exponential value") *is* a float value. If that is not what you expect then tell us what the result should be and why. Otherwise we can only guess. — Martin R, Dec 15 '16 at 10:03
Btw, you asked a similar question some time ago: http://stackoverflow.com/q/36812583/1187415, but never cared to respond on the answer. If an answer does not work for you then you can leave a comment and ask for clarification. — Martin R, Dec 15 '16 at 10:36
Sorry for not replying Martin R, But that question was different , that question was about conversion from float to byte, i think so there is some issue of big endian and little endian, — Nisha Nair, Dec 15 '16 at 10:43
what do you mean by byte data. give one example input and output — Saranjith, Dec 15 '16 at 11:21
[Here](https://stackoverflow.com/questions/56786533/how-to-convert-byte-array-to-float-in-php) is how to convert byte array into float in PHP. — Syed Azhar Abbas, Jun 27 '19 at 10:12

Martin R · Accepted Answer · 2019-08-05T11:41:44.083

22

<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) }))
}

edited Aug 05 '19 at 11:41

answered Dec 15 '16 at 11:57

Martin R

529,903
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Thanks a lot martin..:) – Nisha Nair Dec 15 '16 at 12:29
@MartinR Can you please add reverse of this process, means float to bytes , It's very urgent. – Varun Naharia Oct 04 '17 at 13:10
How this will work when we have 8 bytes and want to convert into 2 floats? Like in objective c I do have ``` union { float f[2]; unsigned char bytes[8]; } u; NSLog(@"mean: %f", u.f[0] ); NSLog(@"variance: %f", u.f[1] ); ```. How this one will look like in swift – Kapil Mar 11 '21 at 04:47
@Kapil: Have a look at https://stackoverflow.com/q/38023838/1187415, where I posted more code about converting values and arrays of values to and from data. – Martin R Mar 11 '21 at 05:30

score 7 · Answer 2 · answered Jul 09 '19 at 15:28

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

score 0 · Answer 3 · answered Dec 15 '16 at 10:23

0

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

answered Dec 15 '16 at 10:23

vbgd

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13
18

i am getting this error Could not cast value of type 'Swift.Int' (0x111dbd1a0) to 'Swift.UInt8' (0x111dbc858). – Nisha Nair Dec 15 '16 at 11:25
What swift version do you use? – vbgd Dec 15 '16 at 12:10

score 0 · Answer 4 · answered Aug 18 '22 at 10:38

0

For 64 bit values the code is:

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

Swift 4+.

answered Aug 18 '22 at 10:38

Kevin Lano

81
2

How to convert bytes to a float value in swift?

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