Swift Native Functions to Have Numbers as Hex Strings

Swift native functions to have numbers as hex strings

As of Swift 2, all integer types have a constructor

init?(_ text: String, radix: Int = default)

so that both integer
to hex string and hex string to integer conversions can be done
with built-in methods. Example:

let num = 1000
let str = String(num, radix: 16)
print(str) // "3e8"

if let num2 = Int(str, radix: 16) {
    print(num2) // 1000
}

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(Old answer for Swift 1:) The conversion from an integer to a hex string can be done with

let hex = String(num, radix: 16)

(see for example How to convert a decimal number to binary in Swift?). This does not require the import of any Framework
and works with any base between 2 and 36.

The conversion from a hex string to an integer can be done with the BSD
library function strtoul() (compare How to convert a binary to decimal in Swift?) if you are willing to import Darwin.

Otherwise there is (as far as I know) no built-in Swift method. Here is an extension
that converts a string to a number according to a given base:

extension UInt {
    init?(_ string: String, radix: UInt) {
        let digits = "0123456789abcdefghijklmnopqrstuvwxyz"
        var result = UInt(0)
        for digit in string.lowercaseString {
            if let range = digits.rangeOfString(String(digit)) {
                let val = UInt(distance(digits.startIndex, range.startIndex))
                if val >= radix {
                    return nil
                }
                result = result * radix + val
            } else {
                return nil
            }
        }
        self = result
    }
}

Example:

let hexString = "A0"
if let num = UInt(hexString, radix: 16) {
    println(num)
} else {
    println("invalid input")
}

How to convert an Int to Hex String in Swift

You can now do:

let n = 14
var st = String(format:"%02X", n)
st += " is the hexadecimal representation of \(n)"
print(st)

0E is the hexadecimal representation of 14

Note: The 2 in this example is the field width and represents the minimum length desired. The 0 tells it to pad the result with leading 0's if necessary. (Without the 0, the result would be padded with leading spaces). Of course, if the result is larger than two characters, the field length will not be clipped to a width of 2; it will expand to whatever length is necessary to display the full result.

This only works if you have Foundation imported (this includes the import of Cocoa or UIKit). This isn't a problem if you're doing iOS or macOS programming.

Use uppercase X if you want A...F and lowercase x if you want a...f:

String(format: "%x %X", 64206, 64206)  // "face FACE"

If you want to print integer values larger than UInt32.max, add ll (el-el, not eleven) to the format string:

let n = UInt64.max
print(String(format: "%llX is hexadecimal for \(n)", n))

FFFFFFFFFFFFFFFF is hexadecimal for 18446744073709551615

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Original Answer

You can still use NSString to do this. The format is:

var st = NSString(format:"%2X", n)

This makes st an NSString, so then things like += do not work. If you want to be able to append to the string with += make st into a String like this:

var st = NSString(format:"%2X", n) as String

or

var st = String(NSString(format:"%2X", n))

or

var st: String = NSString(format:"%2X", n)

Then you can do:

let n = 123
var st = NSString(format:"%2X", n) as String
st += " is the hexadecimal representation of \(n)"
// "7B is the hexadecimal representation of 123"

Cant convert string hex into hexint and then into UIColor

You have to remove the 0x prefix and then specify the radix 16:

let s = "0xe7c79d"
print(Int(s)) // nil

let value = s.hasPrefix("0x")
    ? String(s.dropFirst(2))
    : s
print(Int(value, radix: 16)) // 15189917

Swift native formatting in numeric bases

I read the conversation more closely and it was the 'num' label that broke it. If I use String(123, radix: 16) it works fine. Fractions do NOT work, however. I got a runtime error with String(123.55, radix: 16).

EDIT: As nhgrif points out below, this is now a compiler error, not a runtime error - so it should have never been allowed. This is true as of XCode Version 7.1.1 (7B1005). I believe what is going on is with the num: label on the first parameter it is creating a tuple, then calling the String initializer which converts the tuple to it's string representation. I personally would have expected that to look like this:

let str = String((num: 1, radix: 2));

Which the compiler treats the same way (same result output).

Making a table of hex representations in Swift with compact code

Yes there is. The approach is the same: map each number in the range
0 ... 255 to a string using a hex format:

let hexTable = (0 ..< 256).map { v in String(format: "%02X", v) }

or slightly shorter:

let hexTable = (0 ..< 256).map { String(format: "%02X", $0) }

Result:

["00", "01", "02", ..., "FD", "FE", "FF"]

swift: hexdecimal string to double

Get the integer value from hex string with Int(_:radix:)

let string = "ff14"
let hexValue = Int(string, radix: 16)!

and divide by 65535 (16 bit) to get values between 0.0 and 1.0

let result = Double(hexValue) / 65535

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