How to first 5 objects in Array with computed properties

Question

My project uses an array of computed properties (hourly wages) from which I need to isolate/return only the first 5 objects (wages) in the array. (Note: I have simplified my code for the purpose of isolating this problem, but in so doing it may seem strange that this array is a computed property and not simply a variable. Please accept this oddity as it is necessary in my more complex, complete code.)

Research effort: The Apple Documentation "prefix(upTo:)" would seemingly be a perfect fit for this task. I have referenced the SO question that was asked 7 years ago, but when I apply all the recommendations of all 13 answers, I still get the following errors:

Here is my code with a variety of approaches attempting to accomplish the same result:

class EarningsViewModel: ObservableObject {
    
    var catWageArray: [Double] {
        [81,156,162,166,169,173,177,181,185,192,277,280,282,285,285,285,285,285,285,285,285,285,285,285,285,285,285,285,285,285,285,285,285,285,285,285,285,285,285,285,285]
    }

    var sliceCareerTest =  catWageArray.prefix(5)
    var sliceCareerTest2 =  catWageArray.prefix(upTo: 5)
    var sliceCareer: [Double] {
        catWageArray.prefix(5)}
    var sliceCareer2: [Double] {
        catWageArray.prefix(upTo: 5)}
    
    var n = 5
    var firstFiveSlice = catWageArray[0..<n]
    let firstFiveArray = Array(firstFiveSlice)

It seems to me that the sliceCareer computed property with the error "No 'prefix' candidates produce the expected contextual result type '[Double]'" may be the closest to the actual solution, but I don't understand why there the compiler cannot find prefix candidates.

vadian · Accepted Answer · 2022-12-21 14:44:51Z

2

The proper way is prefix(5) but you have to create an array from the slice (aka SubSequence) and it must be a computed property.

On the other hand catWageArray can be a stored property

class EarningsViewModel: ObservableObject {
    
    var catWageArray: [Double] = [81,156,162,166,169,173,177,181,185,192,277,280,282,285,285,285,285,285,285,285,285,285,285,285,285,285,285,285,285,285,285,285,285,285,285,285,285,285,285,285,285]
    
    
    var sliceCareer: [Double] {
        Array(catWageArray.prefix(5))
    }
...

answered Dec 21, 2022 at 14:44

vadian

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2 Comments

weekendwarrior Over a year ago

Thank you so much! You're my hero! Follow-up question: If I want to remove the first 5 instead of isolate, I've tried using "catWageArray.removeSubrange(0..<5)" but get the following 2 errors: "Cannot use mutating member on immutable value: 'catWageArray' is a get-only property" & "Type '()' cannot conform to 'Sequence'" Feel free to respond with an another "Answer" so I can mark it as "accepted" and "useful"

vadian Over a year ago

This is simply Array(catWageArray.dropFirst(5)). removeSubrange mutates the receiver in place and it works if you follow my suggestion to declare catWageArray as stored property.

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