Making my function calculate average of array Swift

You have some mistakes in your code:

//You have to set the array-type to Double. Because otherwise Swift thinks that you need an Int-array
var votes:[Double] = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

func average(nums: [Double]) -> Double {

    var total = 0.0
    //use the parameter-array instead of the global variable votes
    for vote in nums{
        total += Double(vote)
    }

    let votesTotal = Double(nums.count)
    var average = total/votesTotal
    return average
}

var theAverage = average(votes)

A small one liner, using old fashioned Objective-C KVC translated in Swift:

let average = (votes as NSArray).value(forKeyPath: "@avg.floatValue")

You can also have the sum:

let sum = (votes as NSArray).value(forKeyPath: "@sum.floatValue")

More on this long forgotten gem : https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/KeyValueCoding/CollectionOperators.html

You should use the reduce method to sum your sequence elements as follow:

Xcode Xcode 10.2+ • Swift 5 or later

extension Sequence where Element: AdditiveArithmetic {
    /// Returns the total sum of all elements in the sequence
    func sum() -> Element { reduce(.zero, +) }
}

---

extension Collection where Element: BinaryInteger {
    /// Returns the average of all elements in the array
    func average() -> Element { isEmpty ? .zero : sum() / Element(count) }
    /// Returns the average of all elements in the array as Floating Point type
    func average<T: FloatingPoint>() -> T { isEmpty ? .zero : T(sum()) / T(count) }
}

---

extension Collection where Element: BinaryFloatingPoint {
    /// Returns the average of all elements in the array
    func average() -> Element { isEmpty ? .zero : sum() / Element(count) }
}

---

let votes = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
let votesTotal = votes.sum()                       // 55
let votesAverage = votes.average()                 // 5
let votesDoubleAverage: Double = votes.average()   // 5.5

---

If you need to work with Decimal types its total sum it is already covered by the AdditiveArithmetic protocol extension method, so you only need to implement the average:

extension Collection where Element == Decimal {
    func average() -> Decimal { isEmpty ? .zero : sum() / Decimal(count) }
}

---

---

If you need to sum a certain property of a custom structure we can extend Sequence and create a method that takes a KeyPath as argument to calculate its sum:

extension Sequence  {
    func sum<T: AdditiveArithmetic>(_ predicate: (Element) -> T) -> T {
        reduce(.zero) { $0 + predicate($1) }
    }
}

---

Usage:

struct User {
    let name: String
    let age: Int
}

let users: [User] = [
    .init(name: "Steve", age: 45),
    .init(name: "Tim", age: 50)]

let ageSum = users.sum(\.age) // 95

---

And extend collection to calculate its average:

extension Collection {
    func average<T: BinaryInteger>(_ predicate: (Element) -> T) -> T {
        sum(predicate) / T(count)
    }
    func average<T: BinaryInteger, F: BinaryFloatingPoint>(_ predicate: (Element) -> T) -> F {
        F(sum(predicate)) / F(count)
    }
    func average<T: BinaryFloatingPoint>(_ predicate: (Element) -> T) -> T {
        sum(predicate) / T(count)
    }
    func average(_ predicate: (Element) -> Decimal) -> Decimal {
        sum(predicate) / Decimal(count)
    }
}

---

Usage:

let ageAvg = users.average(\.age)                 // 47
let ageAvgDouble: Double = users.average(\.age)   // 47.5

Simple avarage with filter if needed (Swift 4.2):

let items: [Double] = [0,10,15]
func average(nums: [Double]) -> Double {
    let sum = nums.reduce((total: 0, elements: 0)) { (sum, item) -> (total: Double, elements: Double) in
        var result = sum
        if item > 0 { // example for filter
            result.total += item
            result.elements += 1
        }

        return result
    }

    return sum.elements > 0 ? sum.total / sum.elements : 0
}
let theAvarage = average(nums: items)