Convert bytes/UInt8 array to Int in Swift

Question

How to convert a 4-bytes array into the corresponding Int?

let array: [UInt8] ==> let value : Int

Example:

Input:

Answer 1

There's some good answers here, which is really nice to see ^^ However if you'd like to avoid interacting with the C-interopability API of Swift, then I recommend to take a look at my example. It's also just as generic for all the datatype sizes. Note that MemoryLayout is only being used a sanity check.

Code:

public extension UnsignedInteger {
    init(_ bytes: [UInt8]) {
        precondition(bytes.count <= MemoryLayout.size)

        var value: UInt64 = 0

        for byte in bytes {
            value <<= 8
            value |= UInt64(byte)
        }

        self.init(value)
    }
}

Example usage:

let someBytes = [UInt8](repeating: 0x42, count: 2)
let someValue = UInt16(someBytes)

For little endian support, you need for byte in bytes.reversed() instead.

Explanation:

<<= is the bitwise left shift assignment operator: It shifts the left hand operand (usually a numerical value type) by N bits defined by the right hand operand, for example:

0b00000001 << 7 == 0b10000000

|= is the bitwise or assignment operator: It applies a bitwise or on the left and right hand operands, for example:

0b00000001 | 0b10000000 == 0b10000001

So when you have an array of 2 unsinged bytes and want to convert it a unsinged short you can simply;

let bytes = [UInt8](repeating: UInt8(255), count: 2)
var short: UInt16 = 0

// "add" our first unsinged byte
short |= UInt16(bytes[0])
// our short now looks like this in memory: 0b0000000011111111

// make room for the unsinged byte ;)
short <<= 8 
// our short now looks like this in memory: 0b1111111100000000

// "add" our last unsinged byte
short |= UInt16(bytes[1])
// our short now looks like this in memory: 0b1111111111111111

print(short == UInt16.max)