Group Items By Property in Swift 4 - swift

Question

My data source is
{"events": [{"name":"event
foo","date":"2018-07-21","time":"7:00","am_or_pm":"PM","day":"Saturday","description":"test
"}, {"name":"event
bar","date":"2018-07-21","time":"7:00","am_or_pm":"PM","day":"Saturday","description":"test2"},
{"name":"event
foobar","date":"2018-07-21","time":"11:00","am_or_pm":"PM","day":"Saturday","description":"test3"}]}
I have tried dictionary/arrays, but not really getting close to my wanted result.
Pulling out data into an array:
var times = ["9:00","9:00","11:00"]
var names = ["event foo","event bar","event foobar"]
Desired output:
["9:00", "11:00"]
[["event foo", "event bar"], ["event foobar"]]
Any pointers to do this in Swift is appreciated. My end result is to hope to section a uitableview grouped by time.

Answer 1

If using Swift 4, you can use reduce(into:) and the default value for subscript operator:
guard
let json = (try? JSONSerialization.jsonObject(with: data)) as? [String: [Any]],
let events = json["events"] as? [[String: String]] else {
return
}
let results = events.reduce(into: [String: [String]]()) { result, value in
guard let time = value["time"], let name = value["name"] else { return }
result[time, default: []].append(name)
}
That results in a dictionary:
["11:00": ["event foobar"], "7:00": ["event foo", "event bar"]]
Or, as Vadian suggested, you can use Dictionary(grouping:,by:), but then you have to map it if you only want your name values, resulting in an array of tuples:
let results = Dictionary(grouping: events, by: { $0["time"]! })
.map { ($0.key, $0.value.map { $0["name"]! })}
[("11:00", ["event foobar"]), ("7:00", ["event foo", "event bar"])]
Personally, like Vadian suggested, I'd be inclined to combine date, time, and am_or_pm to build a full Date object and use one of the above patterns. E.g.:
let formatter = DateFormatter()
formatter.dateFormat = "yyyy-MM-dd h:mm a"
formatter.locale = Locale(identifier: "en_US_POSIX")
// if date/time values are in GMT, uncomment the following line:
//
// formatter.timeZone = TimeZone(secondsFromGMT: 0)
let results = events.reduce(into: [Date: [String]]()) { result, value in
guard
let timeString = value["time"],
let dateString = value["date"],
let amPm = value["am_or_pm"],
let date = formatter.date(from: dateString + " " + timeString + " " + amPm),
let name = value["name"] else { return }
result[date, default: []].append(name)
}
or
let results = Dictionary(grouping: events, by: { dictionary -> Date in
let string = dictionary["date"]! + " " + dictionary["time"]! + " " + dictionary["am_or_pm"]!
return formatter.date(from: string)!
})
.map { ($0.key, $0.value.map { $0["name"]! })}
Or, if the web service returned a single ISO 8601/RFC 3339 string representation of the date, time, and am/pm in the JSON, this could be simplified further.

Answer 2

I recommend to decode the JSON with Decodable and create a full date from the components. Then use Dictionary(grouping:by:) to group the array.
First create a DateFormatter (uncomment the time zone line if you need absolute UTC dates)
let dateFormatter : DateFormatter = {
let formatter = DateFormatter()
formatter.locale = Locale(identifier: "en_US_POSIX")
// formatter.timeZone = TimeZone(secondsFromGMT: 0)!
formatter.dateFormat = "yyyy-MM-dd hh:mm a"
return formatter
}()
Create two structs for the root element and the events array. A custom initializer creates the Date instance
struct Root : Decodable {
let events : [Event]
}
struct Event : Decodable {
let date : Date
let name, description : String
private enum CodingKeys: String, CodingKey { case name, date, time, am_or_pm, description}
init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
name = try container.decode(String.self, forKey: .name)
description = try container.decode(String.self, forKey: .description)
let datePortion = try container.decode(String.self, forKey: .date)
let timePortion = try container.decode(String.self, forKey: .time)
let ampm = try container.decode(String.self, forKey: .am_or_pm)
let dateString = "\(datePortion) \(timePortion) \(ampm)"
guard let fullDate = dateFormatter.date(from: dateString) else {
throw DecodingError.dataCorruptedError(forKey: .date,
in: container,
debugDescription: "Date cannot be created")
}
date = fullDate
}
}
Decode the JSON and group the array
let jsonString = """
{"events": [{"name":"event foo","date":"2018-07-21","time":"7:00","am_or_pm":"PM","day":"Saturday","description":"test "}, {"name":"event bar","date":"2018-07-21","time":"7:00","am_or_pm":"PM","day":"Saturday","description":"test2"}, {"name":"event foobar","date":"2018-07-21","time":"11:00","am_or_pm":"PM","day":"Saturday","description":"test3"}]}
"""
do {
let data = Data(jsonString.utf8)
let decoder = JSONDecoder()
let result = try decoder.decode(Root.self, from: data)
let grouped = Dictionary(grouping: result.events, by: { $0.date})
print(grouped)
} catch {
print("error: ", error)
}