getting "fatal error: NSArray element failed to match the Swift Array Element type" when trying to read values from an array of class objects

Question

There is an issue that i can't understand and can not find an answer to: i have this method in a tableViewCOntroller that is calling another viewCOntroller with TableView inside it

override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
    if segue.identifier == "ShowBook" {
        if let selectedIndexPath = tableView.indexPathForSelectedRow {
            let books = categoryStore.allCategories[selectedIndexPath.row].books

            let destinationVC = segue.destinationViewController as! BookViewController
            destinationVC.books = books
        }
    }
}

Then in the BookViewController i have this just for testing:

var books = [Book]()

override func viewDidLoad() {
    super.viewDidLoad()

    print(books[0].name)
}

I can see that the books var is an array that is holding books:

class Book {
var name: String = ""
var id: Int = -1
var categoryId: Int = -1
var bookAuthor: String = ""
var level1Name: String = ""
var level2Name: String = ""
var level3Name: String = ""
var level4Name: String = ""
var levels: Int = -1
var orderKey: Int = -1
var viewCount: Int = -1
var viewLevel: Int = -1
var chapters: [AnyObject] = []

}

so i am getting an array of books with 13 key/value pairs dictionary in the books var

when i'm trying to print any thing, lets say:

print(books[0].name)

I get the error:

fatal error: NSArray element failed to match the Swift Array Element type

and i can't understand why...

p.s The transition is working and i can see the next table but then getting the fatal error

Are you sure there are only Book class objects in your array? — Sander Saelmans
– Sander Saelmans, Commented May 21, 2016 at 10:22
What does the original storage in categoryStore.allCategories[selectedIndexPath.row].books look like? The debugger seems to indicate that you're dealing with dictionaries rather than Books if I'm reading this right. Try printing books[0] first rather than accessing the element. — sas
– sas, Commented May 21, 2016 at 10:52
I don't know how the compiler let you get that far, but you have an NSArray of NSDictionarys. A dictionary entry is not the same as a class instance with 13 properties. — vacawama
– vacawama, Commented May 21, 2016 at 10:52
Try declaring var books = [[NSObject: AnyObject]]() and then print(books[0]["name"]!). — vacawama
– vacawama, Commented May 21, 2016 at 10:57
You have declared books as an array of Book objects but then you have assigned an array of NSDictionary. It looks like you need to create an initialiser for Book that takes a dictionary and then interate your array of NSDictionaries, creating Books and putting them in an array — Paulw11
– Paulw11, Commented May 21, 2016 at 11:03

Peter Hornsby · Accepted Answer · 2016-05-21 13:08:31Z

4

Ok, let's start with your error.

fatal error: NSArray element failed to match the Swift Array Element type

A swift array is complaining that one of it's elements is not what it's expecting. The NSArray element does not match the swift array element.

Now we know that a NSArray can store different types of elements, for example, NSNumber, NSDictionary, NSString... you get the picture. So clearly our issue here is type related.

Now looking at your segue code we can see that we do not actually state what type books is, we let swift's type inference work it out for us. This is where our first sign of the issue occurs. You are expecting an array of [Book] but for some reason you are getting a type of NSArray.

If you make the change from:

let books = categoryStore.allCategories[selectedIndexPath.row].books
let books : [Book] = categoryStore.allCategories[selectedIndexPath.row].books

You will now crash at this line, because you are now stating the type expected.

Full Method:

override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
    if segue.identifier == "ShowBook" {
        if let selectedIndexPath = tableView.indexPathForSelectedRow {
            let books : [Book] = categoryStore.allCategories[selectedIndexPath.row].books

            let destinationVC = segue.destinationViewController as! BookViewController
            destinationVC.books = books
        }
    }
}

This is backed up by your debugger console output, it says that you have an array of 13 NSDictionary types.

This means you need to investigate where you populate the [Book] array - i.e. categoryStore.allCategories

edited May 21, 2016 at 13:08

answered May 21, 2016 at 13:02

Peter Hornsby

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6 Comments

Erez Over a year ago

10x for the direction, i understood it from the comments, and posted the answer with the code to fix this... now every thing is working and i hope it will help other beginner programmers

Peter Hornsby Over a year ago

thanks - I am slow, there was no answer when I started answering the question :) And yes it was covered in the comments I was just answering it explicitly for the readers that follow after us

vadian Over a year ago

Consider that an annotation .. : [Book] ... does not cast the type. For example in case books is [AnyObject] even if it contains only Book objects.

Peter Hornsby Over a year ago

Could add a little more detail to the statement please, I know it does not cast the type, but it does say what the variable's type will be, right?

vadian Over a year ago

No, in this particular case (an array of dictionaries according to the debug console) the annotation does not "convert" the object to an array of Book. Annotations are only useful in a declaration line if the type is ambiguous for example to distinguish OptionSetType e.g. ([.Left, .Center]) from a standard literal Array or to declare a CGFloat rather than the default Double. In all other cases annotations are syntactic sugar and not really needed.

|

Erez · Accepted Answer · 2016-05-21 13:24:24Z

OK, This was a beginner problem that might help others.

The scenario: I have a complex JSON: root is object, for each key in the object there is a numeric value (deferent key but the value has the same structure) and the value is the Category Object! For each category there are many key/Value pairs and one of them is the the Books KEY and the value is array or objects, each one is a book.

The problem: when i deserialised the JSON, i parsed the categories with an array of books, but the books as mentioned in the comments weren't really book instances more array of NSDictionary objects.

So after deserialising the Category i have to add an init method in The Book class to get an NSDictionary and parse it into a Book Object.

After that for each category i have to go over the the NSArray of books and for each one to create an Object go Book and return an Array of Books.

That did the trick and now every thing is working:

API:

// Getting NSArray of NSDictionary that represnets books and convert it into Books array
static func getBooksFromArr(booksArr: NSArray) -> [Book] {
    var books = [Book]()

    for book in booksArr {
        let thisBook = Book(bookDict: book as! NSDictionary)
        books.append(thisBook)
        print(book)
    }

    return books
}

// Parsing the category object
private static func CategoryFromJSONObject(json: [String: AnyObject]) -> Category? {
    guard let
        id = json["Id"] as? Int,
        name = json["Name"] as? String,
        booksArr = json["Books"] as? NSArray else {
            return nil
    }

    let books = getBooksFromArr(booksArr)

    return Category(name: name, id: id, books: books)
}

Book Class:

init(bookDict: NSDictionary){
    self.name = (bookDict["Name"] ?? "") as! String
    self.id = (bookDict["Id"] ?? -1) as! Int
    self.categoryId = (bookDict["CategoryId"] ?? -1) as! Int
    self.bookAuthor = (bookDict["BookAuthor"] ?? "") as! String
    self.level1Name = (bookDict["Level1Name"] ?? "") as! String
    self.level2Name = (bookDict["Level2Name"] ?? "") as! String
    self.level3Name = (bookDict["Level3Name"] ?? "") as! String
    self.level4Name = (bookDict["Level4Name"] ?? "") as! String
    self.levels = (bookDict["Levels"] ?? -1) as! Int
    self.orderKey = (bookDict["OrderKey"] ?? -1) as! Int
    self.viewCount = (bookDict["ViewCount"] ?? -1) as! Int
    self.viewLevel = (bookDict["ViewLevel"] ?? -1) as! Int
    self.chapters = (bookDict["Chapters"] ?? []) as! [AnyObject]
}

Now i will have to do the same for the chapters if i want to compose books to hold the chapters.

I didn't write the REST API so don't ask me why they did it as so, but now every thing is working and that is the point of it.

Ivan · Accepted Answer · 2019-01-29 13:47:50Z

0

In my case, it was just casting to the wrong array type in the code above the problem...

answered Jan 29, 2019 at 13:47

Ivan

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