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

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]

Looking at your example results, it seems you want the String representation of your Date to be the key (not the Date itself). Also, if Date is not a typealias for String, the Item default initializer calls when instantiating the data array wont compile. — dfrib
– dfrib, Commented Feb 15, 2017 at 18:03
I actually want date, but it was easier to write it as a string — Adrian
– Adrian, Commented Feb 15, 2017 at 18:05
The initializer from String to Date on your example will not work. Do you mean declaring Item.date as String instead of Date? — nbloqs
– nbloqs, Commented Feb 15, 2017 at 18:05
@Kobe just gave an example of date for better understanding, right? — Mr. Xcoder
– Mr. Xcoder, Commented Feb 15, 2017 at 18:06

dfrib · Accepted Answer · 2017-02-15 18:20:39Z

3

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]

edited Feb 15, 2017 at 18:20

answered Feb 15, 2017 at 18:09

dfrib

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1 Comment

dfrib Over a year ago

@Xcoder123 dictionaries are non-ordered collections, so the "order" shown holds no value here.

Imanou Petit · Accepted Answer · 2017-06-13 08:28:37Z

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]

Community · Accepted Answer · 2020-06-20 09:12:55Z

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
}

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

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#1 Using `Dictionary` `subscript(_:default:)`

#2 Using `Dictionary` `init(_:uniquingKeysWith:)` initializer

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Solution:

Helper methods:

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Collectives™ on Stack Overflow

3 Answers 3

1 Comment

#1 Using Dictionary subscript(_:default:)

#2 Using Dictionary init(_:uniquingKeysWith:) initializer

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Solution:

Helper methods:

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#1 Using `Dictionary` `subscript(_:default:)`

#2 Using `Dictionary` `init(_:uniquingKeysWith:)` initializer