HowTo save Swift CoreData values into an array

Question

I'm new in Swift and CoreData and I have a question on how to read data that was stored in CoreData (NsManagedObject) and save it to an data array which can than be used as input for a SwiftUI Line Chart.

My test data model is simple:

@NSManaged public var date: Date?
@NSManaged public var value: Double

I'm able to read the number of values by using this code:

private func fetchItem() {
        withAnimation {
            let fetchRequest: NSFetchRequest<Rate> = Rate.fetchRequest()
            do {
                let fetchedResults = try viewContext.fetch(fetchRequest)
                print("#Entities: \(fetchedResults.count)")
                
            } catch {
                print(error)
              }
        }
    }

Now I need the double values from Core Data stored in an array. For the Line Chart demo (iLineChart) I have the following test data array:

let data = (1...100).map( {_ in Double.random(in: -1...10)} )

Question:

How can I create an array with double values from the NSManagedObject values?

Regards, Ralf

Larme · Accepted Answer · 2021-04-16 08:45:44Z

5

You want to convert an array of Rate (ie [Rate]) into an Array of Double (ie Double), where it will be filled with the value of each Rate.

That's a job for map()! It does exactly that: iterate over the initial array, for each element (here a Rate element) return what you want to extract from it (here just its property value).

Quick answer and "Swifty" one:

let data = fetchedResult.map { $0.value }

More explicit:

let data = fetchedResult.map { aRate in 
    return aRate.value
}

Manually done:

var data: Double = []
for aRate in fetchedResults { 
    data.append(aRate.value)
}

Note: The map is Swifty, in oneline, BUT no one should criticize the use of the manuel for loop. That's basic algorithm, and understanding them is a solid foundation on how to develop. So if it's still your level of knowledge, I'd recommend to use the for loop. Because if you need to change the output/result of the map, will you be able to do so? Let's say the value + an offset of 30 on the value, and all in absolute values?

answered Apr 16, 2021 at 8:45

Larme

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1 Comment

ralfb Over a year ago

THANKS a lot - you saved my day. I have used the for loop and it works as described. Have a nice day.

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