Append Values in NSSet To An Array of Type [String]

Question

I am attempting to get the values from an NSSet in core data and append those values to an array of type String.

func addStepsToArray() {
    if let steps = entity.steps {
        for i in steps {
            recipeStep.append(String(i))
        }
    }
}

entity.steps is the list of steps tied to a core data entity. This is an NSSet. I am trying to copy those values to an array of type String.

@State var recipeStep: [String]

When trying to do this in my for in loop, I receive the following error: No exact matches in call to initializer

If I remove the conversion of "I" to String, I receive the following error:

Cannot convert value of type NSSet.Element (aka Any) to expected argument type String

Any idea on how to get this to work?

Alexander · Accepted Answer · 2021-07-06 23:20:15Z

3

NSSet is defined in Objective C, which didn't have generics. It's an untyped collection, so you don't statically know anything about its elements.

As you've noticed, your i variable isn't a String, it's an Any.

You're confusing type coercion ("casting") with type conversion. If i were a Double, you could call String(i) to invoke an initializer which takes a double, and processes into a String.

You tried something similar by calling String(i), where you're making the Swift compiler find an initializer on String with the signitiure init(_: Any).

There is no such initializer. And besides, that's not what you want. You don't want to create a new String from a different kind of value. You already have a string, it's just "hidden" behind an Any reference.

What you're looking for is to do a down-cast, from Any to String:

func addStepsToArray() {
    if let steps = entity.steps {
        for i in steps {
            guard let step = i as? String else {
                fatalError("Decide what to do if the value isn't a String.")
            }
            recipeStep.append(i as String)
        }
    }
}

I'll warn you though, there are several issues/blemishes with this code:

You're using a for loop to do what is ultimately just a mapping operation
Your computation doesn't return its ouput, and instead indirectly achieves its goal through a side-effect on recipeStep
Your computation doesn't take a its input as a parameter, and instead indirectly achieves its goal through a side-effect on entity
i is conventionally expected to be an integer index of a for loop iterating over a sequence of numbers. Here it's an Any (a String at runtime)

Here's what I would suggest instead:

func getRecipeSteps(from entity: MyEntityType) -> [String] {
    guard let steps = entity.steps else { return [] }

    return steps.map { step in
        guard let stringStep = step as? String else { 
            fatalError("Decide what to do if the value isn't a String.")
        }

        return step
    }
}

Then in the rest of your code (and your tests), you can write self.recipeSteps = getRecipeSteps(from: myEntity). Elegant!

If you're certain that these entity.steps values can only ever be strings, then you can boil this down to a single map with a force-cast:

func getRecipeSteps(from entity: MyEntityType) -> [String] {
    entity.steps?.map { $0 as! String } ?? []
}

edited Jul 6, 2021 at 23:20

answered Jul 5, 2021 at 22:36

Alexander

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8 Comments

nickreps Over a year ago

Thanks for the extremely detailed answer. I have tried all three approaches, but unfortunately the app crashes when landing on the page where the function executes (I am executing via .onAppear). The error is as follows: "Could not cast value of type 'RecipeTracker.StepsEntity' to 'NSString'". It looks like it is having trouble with the type casting. Thoughts?

Alexander Over a year ago

Well you were operating on a misapprehension. You have some kind of StepsEntity which is clearly not a String.

nickreps Over a year ago

StepsEntity itself is an entity that contains multiple attributes, but each step attribute within that entity is a String. Maybe if I tried something like entity.steps?.allObjects as? [StepsEntity]) and incorporated that into a map and it would return the correct data. I will give this a try later. I think the issue is it is trying to return all attributes in the entity, when it should only be returning the step attribute.

Alexander Over a year ago

@nickreps So you probably want entity.steps.map { step in steps.theStringValueYouWant }. You haven't provided enough details in your question.

nickreps Over a year ago

I ended up getting it to work using the following: func addSteps() { if let recipeSteps = (entity.steps?.allObjects as? [StepsEntity])?.sorted(by: { $0.stepNumber < $1.stepNumber }) { for index in recipeSteps { recipeStep.append(index.step ?? "") stepNumber.append(Int(index.stepNumber)) I am still going to mark yours as the accepted answer, as yours is correct based on what I described.

|

Alexander · Accepted Answer · 2021-07-06 01:03:34Z

0

Just convert directly：

let set = Set(["1", "2", "3"])
let array = Array(set)
DDLog(set)//Set<String>)
DDLog(array)//[String]

answered Jul 6, 2021 at 1:03

Alexander

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