Swift: extract float from byte data

问题

I'm looking for a robust and elegant way to extract four big-endian bytes from an array as a Float.

I can get a UInt32 with the bits via something like this:

let data: [Byte] = [0x00, 0x00, 0x00, 0x40, 0x86, 0x66, 0x66, 0x00]
let dataPtr = UnsafePointer<Byte>(data)
let byteOffset = 3
let bits = UnsafePointer<UInt32>(dataPtr + byteOffset)[0].bigEndian

But I can't figure out a good way to convert this into a Float in Swift.

For example, in Java:

float f = Float.intBitsToFloat(bits)

or in C:

float f = *(float *)&bits;

I tried casting the dataPtr to a float UnsafePointer, but then the endianness is a problem.

Any suggestions?

回答1:

The floating point types have a static _fromBitPattern that will return a value. <Type>._BitsType is a type alias to the correctly sized unsigned integer:

let data: [Byte] = [0x00, 0x00, 0x00, 0x40, 0x86, 0x66, 0x66, 0x00]
let dataPtr = UnsafePointer<Byte>(data)
let byteOffset = 3
let bits = UnsafePointer<Float._BitsType>(dataPtr + byteOffset)[0].bigEndian
let f = Float._fromBitPattern(bits)

You don't see that method in auto-completion, but it's a part of the FloatingPointType protocol. There's an instance method that will give you back the bits, called ._toBitPattern().

回答2:

The equivalent Swift code is

let flt = unsafeBitCast(bits, Float.self)

which gives 4.2 with your data.

回答3:

Here's the solution for Swift 3:

Float(bitPattern: bits)

来源：https://stackoverflow.com/questions/26976579/swift-extract-float-from-byte-data