Extension for average to return Double from Numeric generic

问题

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.

回答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

来源：https://stackoverflow.com/questions/54391361/extension-for-average-to-return-double-from-numeric-generic