Decomposing a number into powers of 2 - swift

Question

Hi I need to decompose a number into powers of 2 in swift 5 for an iOS app I'm writing fro a click and collect system.
The backend of this system is written in c# and uses the following to save a multi-pick list of options as a single number in the database eg:
choosing salads for a filled roll on an order system works thus:
lettuce = 1
cucumber = 2
tomato = 4
sweetcorn = 8
onion = 16
by using this method it saves the options into the database for the choice made as (lettuce + tomato + onion) = 21 (1+4+16)
at the other end I use a c# function to do this thus:
for(int j = 0; j < 32; j++)
{
int mask = 1 << j;
}
I need to convert this function into a swift 5 format to integrate the decoder into my iOS app
any help would be greatly appreciated

Answer 1

In Swift, these bit fields are expressed as option sets, which are types that conform to the OptionSet protocol. Here is an example for your use case:
struct Veggies: OptionSet {
let rawValue: UInt32
static let lettuce = Veggies(rawValue: 1 << 0)
static let cucumber = Veggies(rawValue: 1 << 1)
static let tomato = Veggies(rawValue: 1 << 2)
static let sweetcorn = Veggies(rawValue: 1 << 3)
static let onion = Veggies(rawValue: 1 << 4)
}
let someVeggies: Veggies = [.lettuce, .tomato]
print(someVeggies) // => Veggies(rawValue: 5)
print(Veggies.onion.rawValue) // => 16
OptionSets are better than just using their raw values, for two reasons:
1) They standardize the names of the cases, and gives a consistent and easy way to interact with these values
2) OptionSet derives from the SetAlgebra protocol, and provides defaulted implementations for many useful methods like union, intersection, subtract, contains, etc.
I would caution against this design, however. Option sets are useful only when there's a really small number of flags (less than 64), that you can't forsee expanding. They're really basic, can't store any payload besides "x exists, or it doesn't", and they're primarily intended for use cases that have very high sensitivity for performance and memory use, which quick rare these days. I would recommend using regular objects (Veggie class, storing a name, and any other relevant data) instead.

Answer 2

You can just use a while loop, like this :
var j = 0
while j < 32 {
var mask = 1 << j
j += 1
}
Here is a link about loops and control flow in Swift 5.

Answer 3

Hi I figured it out this is my final solution:
var salads = "" as String
let value = 127
var j=0
while j < 256 {
let mask=1 << j
if((value & mask) != 0) {
salads.append(String(mask) + ",")
}
j += 1
}
salads = String(salads.dropLast()) // removes the final ","
print(salads)
This now feeds nicely into the in clause in my SQL query, thanks you all for your help! :)