how do i get an average value for scores in a table view (swift)?

Question

I want to display an average for all values that are inputted into an Entity. Basically, one would press add on the TableView and a pop up would appear asking for a score, which would then add that “exam” to the tableview. I want to be able to add up all the scores and receive an average for them which i can then add to a label. I’ve tried following some other tutorial but it gives me an error.

https://i.stack.imgur.com/P7exB.jpg https://i.stack.imgur.com/WWITI.jpg

The images above are for context.

var i = 0
var sum = 0

    for i in 0...methodsExam.count {
        let title = methodsExam[i]
        let str : String = title.value(forKey: "score") as! String
        sum = sum + Int(str)!
    }
    let avg = sum/methodsExam.count
    
    averageScore.text = "Average: \(avg)"

Check your loop again and read about ranges. `for i in 0...array.count` *includes* the upper bound. — Martin R, Sep 29 '20 at 07:22
I also suggest to learn how to *debug* a problem. If you get an “index out of range” error then set a breakpoint and inspect the index. Or add print statements showing the indices. You should see the problem quickly. — Martin R, Sep 29 '20 at 07:23
@JoakimDanielson i’m not sure what you mean, i have written for i in... is that what you were trying to say. — aadi sach, Sep 29 '20 at 07:43
Sorry, I mean a for in loop without using an index. `for item in methodsExam {` — Joakim Danielson, Sep 29 '20 at 09:35
@JoakimDanielson don’t think that worked for me. could you elaborate on what you mean? — aadi sach, Sep 29 '20 at 10:20
@JoakimDanielson Your code would be `for score in methodsExam {}`. You should also ensure your array isn't empty — Harry J, Sep 29 '20 at 12:17

Joakim Danielson · Answer 1 · 2020-09-29T12:55:56.503

Assuming methodsExam is an array of [MethodsExam] you can change the loop to

var sum = 0.0
for item in methodsExam {
    if let scoreStr = item.score, let score = Double(scoreStr) {
        sum += score
    }
}
let average = sum / Double(methodsExam.count)

I am using Double here for more precision.

This could also be done with a high-order function

let average = methodsExam.reduce(into: 0.0) {
    if let scoreStr = $1.score, let score = Double(scoreStr) { $0 += score }
} / Double(methodsExam.count)

Since we are dividing with the array count we must also check that the array isn't empty to avoid division by 0, so we can put the code into a function and do this

func calculateAverage(_ array: [MethodsExam]) -> Double {
    guard !array.isEmpty else { return 0.0 }

    return  array.reduce(into: 0.0) {
        if let scoreStr = $1.score, let score = Double(scoreStr) { $0 += score }
    } / Double(array.count)
}

One caveat, I assumed that score can be nil because of MethodsExam being an NSManagedObject and not because it will hold nil values. If it indeed can hold nil values then you need to consider what to calculate the average on, all values in the array or only non-nil values.

thank you so much for this, however, no matter what i do i seem to be getting “nan” returned. am i doing something wrong? — aadi sach, Sep 30 '20 at 00:42
the code for methodsExam is this: var methodsExam : MethodsExam = [] — aadi sach, Sep 30 '20 at 01:00
@aadisach No that is not correct, it should be `var methodsExam : [MethodsExam] = []`. Your code should generate a complier error? — Joakim Danielson, Sep 30 '20 at 07:49

how do i get an average value for scores in a table view (swift)?

1 Answers1