Sort Objects in Array by date

问题:

I have an array containing an object called HistoryObject and it has properties such as "date", "name", etc.

I am sorting the array like so:

 let sortedArray = HistoryArray.sort({ $0.date.compare($1.date) == NSComparisonResult.OrderedDescending})

which is supposed to sort the date from newer to oldest. For example:

• Jun 30, 2016
• Jun 29, 2016

etc..

But when my array contains "Jul 2, 2016" the sorted array becomes:

• Jun 30, 2016
• Jun 29, 2016
• Jul 2, 2016

Where "Jul 2, 2016" should be on top after sorting, now it's on the bottom? How can I fix this problem?

回答1:

Using Swift 4 & Swift 3

let testArray = ["25 Jun, 2016", "30 Jun, 2016", "28 Jun, 2016", "2 Jul, 2016"]
var convertedArray: [Date] = []

var dateFormatter = DateFormatter()
dateFormatter.dateFormat = "dd MM, yyyy"// yyyy-MM-dd"

for dat in testArray {
    let date = dateFormatter.date(from: dat)
    if let date = date {
        convertedArray.append(date)
    }
}

var ready = convertedArray.sorted(by: { $0.compare($1) == .orderedDescending })

print(ready)

Using Swift 2

For example you have the array with dates and another 1 array, where you will save the converted dates:

var testArray = ["25 Jun, 2016", "30 Jun, 2016", "28 Jun, 2016", "2 Jul, 2016"]
var convertedArray: [NSDate] = []

After that we convert the dates:

var dateFormatter = NSDateFormatter()
dateFormatter.dateFormat = "dd MM, yyyy"// yyyy-MM-dd"

for dat in testArray {
    var date = dateFormatter.dateFromString(dat)
    convertedArray.append(date!)
}

And the result:

var ready = convertedArray.sort({ $0.compare($1) == .OrderedDescending })

print(ready)

回答2:

For Swift 3

var testArray = ["25 Jun, 2016", "30 Jun, 2016", "28 Jun, 2016", "2 Jul, 2016"]
var convertedArray: [Date] = []

var dateFormatter = DateFormatter()
dateFormatter.dateFormat = "dd/MM/yyyy"

for dat in testArray {
    var date = dateFormatter.date(from: dat)
    convertedArray.append(date!)
}

//Approach : 1
convertedArray.sort(){$0 < $1}    

//Approach : 2
convertedArray.sorted(by: {$0.timeIntervalSince1970 < $1.timeIntervalSince1970})

print(convertedArray)

回答3:

Avoiding extra variable of convertedArray

Using Swift 4 & Swift 3

let testArray = ["25 Jun, 2016", "30 Jun, 2016", "28 Jun, 2016", "2 Jul, 2016"]

var dateFormatter = DateFormatter()
dateFormatter.dateFormat = "dd MM, yyyy"// yyyy-MM-dd"

var ready = convertedArray.sorted(by: { dateFormatter.date(from:$0).compare(dateFormatter.date(from:$1)) == .orderedDescending })

print(ready)

回答4:

You have an array historyArray that contains an array of HistoryObject. Each HistoryObject contains a date string in the form "MM dd, yyyy"

Edit:

(You want to sort your history objects by their date values. It is a bad idea to try to sort objects with date strings by those date strings, since you have to convert the date strings to Cocoa Date objects for every comparison, so you end up converting the dates to date objects over and over and over. In a benchmark I did, this causes the sort to run 1200X slower than if you batch-convert your date strings to Date objects before sorting, as outlined below.)

In order to do that efficiently you need to first get Date values for all of the objects. One way to do that would be to add a lazy Date var to your HistoryObject that is calculated from the date string. If you don't want to do that you can:

1. Map your array of history objects to an array of Date objects using a DateFormatter.
2. Use the zip() function to combine the array of history objects and the array of date objects into an array of tuples.
3. Sort the array of tuples.
4. Map the array of tuples back to an array of history objects.

The code to do that might look something like this:

Version 1

var dateFormatter = DateFormatter()
dateFormatter.dateFormat = "MM dd, yyyy"

//I don't know what your HistoryObject looks like, so I'll fake it.
struct HistoryObject: CustomStringConvertible {
    let dateString: String
    let value: Int

    var description: String {
        return "date: \(dateString), value: \(value)"
    }
}

//Create an array of date strings.
let testArray = ["Jun 25, 2016", "Jun 30, 2016", "Jun 28, 2016", "Jul 2, 2016"]

//Use the array of date strings to create an array of type [HistoryObject]
let historyArray: [HistoryObject] = testArray.map {
    let value = Int(arc4random_uniform(1000))
    return HistoryObject(dateString: $0, value: value)
}

print("\n-----> Before sorting <-----")
historyArray.forEach { print($0) }

//Create an array of the `Dates` for each HistoryObject
let historyDates: [Date] = historyArray.map { dateFormatter.date(from: $0.dateString)!
}

//Combine the array of `Dates` and the array of `HistoryObjects` into an array of tuples
let historyTuples = zip(historyArray, historyDates)

//Sort the array of tuples and then map back to an array of type [HistoryObject]
let sortedHistoryObjects = historyTuples.sorted { $0.1 > $1.1}
    .map {$0.0}

print("\n-----> After sorting <-----")
sortedHistoryObjects.forEach { print($0) }

If you add a lazy var date to your HistoryObject the sorting code is a LOT simpler:

Version 2:

//I don't know what your HistoryObject looks like, so I'll fake it.
class HistoryObject: CustomStringConvertible {
    let dateString: String
    lazy var date: Date = { dateFormatter.date(from: self.dateString)! }()
    let value: Int

    var description: String {
        return "date: \(dateString), value: \(value)"
    }

    init(dateString: String, value: Int) {
        self.dateString = dateString
        self.value = value
    }
}

//Create an array of date strings.
let testArray = ["Jun 25, 2016", "Jun 30, 2016", "Jun 28, 2016", "Jul 2, 2016"]

//Use the array of date strings to create an array of type [HistoryObject]
let historyArray: [HistoryObject] = testArray.map {
    let value = Int(arc4random_uniform(1000))
    return HistoryObject(dateString: $0, value: value)
}

print("\n-----> Before sorting <-----")
historyArray.forEach { print($0) }

let sortedHistoryArray = historyArray.sorted { $0.date > $1.date }
print("\n-----> After sorting <-----")
sortedHistoryArray.forEach { print($0) }