How to group array data based on Date item into a dictionary - arrays

Question

I have an array of Item objects:
struct Item {
let date: Date
let amount: Double
}
How can I group this items into a dictionary with Date as key and sum of amount for that date in Swift 3 ?
Here is an example:
let data = [Item(date:"2017.02.15", amount: 25),
Item(date:"2017.02.14", amount: 50),
Item(date:"2017.02.11", amount: 35),
Item(date:"2017.02.15", amount: 15)]
Result should be:
["2017.02.15": 40, "2017.02.14": 50, "2017.02.11": 35]

Answer 1

Given we fix your example up somewhat (using String as Date example), you could simply pass over the elements in data and add to (and optionally increase) the corresponding key-value pair in the dict.
struct Item {
let date: String
let amount: Double
}
let data = [Item(date: "2017.02.15", amount: 25),
Item(date: "2017.02.14", amount: 50),
Item(date: "2017.02.11", amount: 35),
Item(date: "2017.02.15", amount: 15)]
var dict: [String: Double] = [:]
for item in data {
dict[item.date] = (dict[item.date] ?? 0) + item.amount
}
print(dict) // ["2017.02.14": 50.0, "2017.02.15": 40.0, "2017.02.11": 35.0]

Answer 2

With Swift 4, according to your needs, you may choose one of the following solutions in order to solve your problem.
#1 Using Dictionary subscript(_:default:)
Dictionay has a subscript called subscript(_:default:). subscript(_:default:) has the following declaration:
subscript(key: Dictionary.Key, default defaultValue: #autoclosure () -> Dictionary.Value) -> Dictionary.Value { get set }
Accesses the element with the given key, or the specified default value, if the dictionary doesn’t contain the given key.
The following Playground example shows how to use subscript(_:default:) in order to solve your problem:
struct Item {
let date: String
let amount: Double
}
let data = [
Item(date:"2017.02.15", amount: 25),
Item(date:"2017.02.14", amount: 50),
Item(date:"2017.02.11", amount: 35),
Item(date:"2017.02.15", amount: 15)
]
var dictionary = [String: Double]()
for item in data {
dictionary[item.date, default: 0.0] += item.amount
}
print(dictionary) // ["2017.02.11": 35.0, "2017.02.15": 40.0, "2017.02.14": 50.0]
#2 Using Dictionary init(_:uniquingKeysWith:) initializer
Dictionay has a init(_:uniquingKeysWith:) initializer. init(_:uniquingKeysWith:) has the following declaration:
init<S>(_ keysAndValues: S, uniquingKeysWith combine: (Value, Value) throws -> Value) rethrows where S : Sequence, S.Element == (Key, Value)
Creates a new dictionary from the key-value pairs in the given sequence, using a combining closure to determine the value for any duplicate keys.
The following Playground example shows how to use init(_:uniquingKeysWith:) in order to solve your problem:
struct Item {
let date: String
let amount: Double
}
let data = [
Item(date:"2017.02.15", amount: 25),
Item(date:"2017.02.14", amount: 50),
Item(date:"2017.02.11", amount: 35),
Item(date:"2017.02.15", amount: 15)
]
let tupleArray = data.map({ ($0.date, $0.amount) })
let dictonary = Dictionary(tupleArray, uniquingKeysWith: { (current, new) in
current + new
})
//let dictonary = Dictionary(tupleArray, uniquingKeysWith: +) // also works
print(dictonary) // prints ["2017.02.11": 35.0, "2017.02.15": 40.0, "2017.02.14": 50.0]

Answer 3

I thought this solution may be better since your property date actually is a Date
Solution:
struct Item {
let date: Date
let amount: Double
}
var data = [Item(date:createDate(stringDate: "2017.02.15"), amount: 25),
Item(date:createDate(stringDate: "2017.02.14"), amount: 50),
Item(date:createDate(stringDate: "2017.02.11"), amount: 35),
Item(date:createDate(stringDate: "2017.02.15"), amount: 15)]
data = sumAmounts(data)
print(data)
//[
// Item(date: 2017-02-15 04:00:00 +0000, amount: 40.0),
// Item(date: 2017-02-14 04:00:00 +0000, amount: 50.0),
// Item(date: 2017-02-11 04:00:00 +0000, amount: 35.0)
//]
Helper methods:
func createDate(stringDate: String) -> Date {
let formatter = DateFormatter()
formatter.dateFormat = "yyyy.MM.dd"
return formatter.date(from: stringDate)!
}
func sumAmounts(_ data: [Item]) -> [Item] {
var dict = [Double: Double]()
data.forEach { item in
let key = item.date.timeIntervalSince1970
var amount = dict[key] ?? 0
amount += item.amount
dict[key] = amount
}
var newData = [Item]()
for (key, val) in dict {
newData.append(Item(date: Date.init(timeIntervalSince1970: key), amount: val))
}
return newData
}