How to calculate averages by date/month/year from Array in Swift3 - arrays

Question

I have an Array of Dictionaries.
The Dictionary is like ["date":"2017-07-03T10:12:00", "amount":2.1].
EDIT
Let's say, I have the following array.
let dic1 = ["date":"2017-07-03T10:12:00", "amount":2.0]
let dic2 = ["date":"2017-07-03T11:15:03", "amount":4.0]
let dic3 = ["date":"2017-07-03T17:05:04", "amount":1.0]
let dic4 = ["date":"2017-07-04T09:05:03", "amount":3.0]
let dic5 = ["date":"2017-07-04T12:01:22", "amount":5.0]
let dic6 = ["date":"2017-07-04T19:01:01", "amount":2.0]
let array = [dic1, dic2, dic3, dic4, dic5, dic6]
Now I'm trying to calculate the following result from Array.
"date":"2017-07-03", "average":3.5
"date":"2017-07-04", "average":5.0
"date":"2017-07", "average":2.82..
"date":"2017", "average":2.82..
I could get the average of all data with the following code but I'm stuck now.
let average = array.flatMap({ $0["amount"] as? Double }).reduce(0, +) / Double(array.count)

Answer 1

Suggestion using a custom struct including a date formatter to convert the ISO8601 string to Date
struct Item {
let date : Date
let amount : Double
static let dateFormatter : ISO8601DateFormatter = {
let formatter = ISO8601DateFormatter()
formatter.formatOptions = [.withFullDate, .withTime, .withColonSeparatorInTime]
return formatter
}()
init(date: String, amount: Double) {
self.date = Item.dateFormatter.date(from: date)!
self.amount = amount
}
}
Populate the array
let items = [Item(date : "2017-07-03T10:12:00", amount :2.0),
Item(date : "2017-07-03T11:15:03", amount :4.0),
Item(date : "2017-07-03T17:05:04", amount :1.0),
Item(date : "2017-07-04T09:05:03", amount :3.0),
Item(date : "2017-07-04T12:01:22", amount :5.0),
Item(date : "2017-07-04T19:01:01", amount :2.0)]
Create a function to take the array and specific date components as parameters
func average(of items: [Item], matching components: DateComponents) -> Double
{
let filteredDates = items.filter { Calendar.current.date($0.date, matchesComponents: components) }
return filteredDates.map{ $0.amount }.reduce(0.0, +) / Double(filteredDates.count)
}
Now filter the items by date components and return the average value
average(of: items, matching: DateComponents(year: 2017, month: 7, day: 3))
average(of: items, matching: DateComponents(year: 2017, month: 7, day: 4))
average(of: items, matching: DateComponents(year: 2017, month: 7))
average(of: items, matching: DateComponents(year: 2017))

Answer 2

You should filter your array and calculate the average of the filtered values. This example is for the year 2017, but you can apply the same method for other values as well.
let filteredArray = array.filter { Calendar.current.component(.year, from: $0) == 2017 }
let average = (filteredArray.map { $0 as? Double ?? 0 }).reduce(0, +) / Double(filteredArray.count)