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Suppose I create a protocol and structure for a Column of homogeneously typed data:

protocol Columnizable {
    associatedtype Item

    var name: String { get }
    var values: [Item] { get }

}

struct Column<T>: Columnizable {

    var name: String
    var values = [T]()

}

I would like to create a Protocol extension that allows Numeric to have an average function that could compute the average of values if the type conforms to Numeric protocol- for instance, namely Double and Int

extension Columnizable where Item: Numeric {

    func average() -> Double {
        return Double(sum()) / values.count
    }

    func sum() -> Item {
        return values.reduce(0, +)
    }

}

My attempt at the average function cannot be compiled because of:

Cannot invoke initializer for type 'Double' with an argument list of type '(Self.item)'

Attempts to cast to Double do not work. Any advice for best practices here would be appreciated.

Robert F. Dickerson
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    Possibly helpful: [Is there a way to convert any generic Numeric into a Double?](https://stackoverflow.com/q/53771879/1187415) – Martin R Jan 27 '19 at 18:23
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    If I created a `Complex` type that conformed to `Numeric` (which is quite straightforward to do), what would you want to happen here? Thinking through that case will likely lead you to the better answer. – Rob Napier Jan 27 '19 at 18:33
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    @MartinR https://stackoverflow.com/a/28288619/2019221 – Callam Jan 27 '19 at 19:18
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    Thank you guys, @Callam that solution was right on point. I did not know about `BinaryInteger` and `BinaryFloatingPoint`. – Robert F. Dickerson Jan 27 '19 at 19:46

1 Answers1

1

I needed to use the BinaryInteger or BinaryFloatingPoint protocols since they can easily be transformed to a Double. As @rob napier called out, that a Complex type would not be Double convertible.

extension Columnizable where Item: BinaryInteger {
    var average: Double {
        return Double(total) / Double(values.count)
    }
}

extension Columnizable where Item: BinaryFloatingPoint {
    var average: Item {
        return total / Item(values.count)
    }
}

stackoverflow.com/a/28288619/2019221

Robert F. Dickerson
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