How to convert 4 bytes to a Swift float?

问题

I'm writing a MsgPack parser in Swift as a way to learn the language. It doesn't feel very well suited to the task but I've been making progress. Now I've hit a block where I can't seem to convince it to convert 4 bytes into a float.

var bytes:Array<UInt8> = [0x9A, 0x99, 0x99, 0x41] //19.20000

var f:Float = 0

memccpy(&f, &bytes, 4, 4)

print(f)

In the playground I get:

fatal error: Can't unwrap Optional.None Playground execution failed: error: Execution was interrupted, reason: EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0).

Any ideas what to try next?

回答1:

Drop the & on &bytes. bytes is an array.

    var bytes:Array<UInt8> = [0x9A, 0x99, 0x99, 0x41] //19.20000

    var f:Float = 0.0

    memccpy(&f, bytes, 4, 4) // as per OP. memcpy(&f, bytes, 4) preferred

    println ("f=\(f)")// f=19.2000007629395

Update Swift 3

memccpy does not seem to work in Swift 3. As commentators have said, use memcpy :

import Foundation
var bytes:Array<UInt8> = [0x9A, 0x99, 0x99, 0x41] //19.20000

var f:Float = 0.0

/* Not in Swift 3
 memccpy(&f, bytes, 4, 4) // as per OP.

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

memcpy(&f, bytes, 4) /

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

回答2:

public func parseInt32(bytes:[UInt8], offset:Int)->Int32{

    var pointer = UnsafePointer<UInt8>(bytes)
    pointer = pointer.advancedBy(offset)

    let iPointer =  UnsafePointer<Int32>(pointer)
    return iPointer.memory

}

public func parseFloat32(bytes:[UInt8], offset:Int)->Float32{
    var pointer = UnsafePointer<UInt8>(bytes)
    pointer = pointer.advancedBy(offset)

    let fPointer =  UnsafePointer<Float32>(pointer)
    return fPointer.memory

}

来源：https://stackoverflow.com/questions/24359333/how-to-convert-4-bytes-to-a-swift-float

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