How to sort a List<String> of formatted dates in dart?

Question

I have tried this on dartpad,

void main() {
  final List<String> dates=["24-06-2021","27-05-2021","21-04-2021","29-07-2021","15-12-2021"];
  var sorted = dates..sort((a,b) => a.compareTo(b));
  print(sorted);
}

Output : [15-12-2021, 21-04-2021, 24-06-2021, 27-05-2021, 29-07-2021]
Desired : [21-04-2021, 27-05-2021, 24-06-2021, 29-07-2021, 15-12-2021]

Any help is appreciated.

Solution 1

import 'package:intl/intl.dart';

List<String> getSortedDates(List<String> dates){

  //Formatting for acceptable DateTime format
  DateFormat formatter = DateFormat("dd-MM-yyyy");

  //Mapping to convert into appropriate dateFormat
  List<DateTime> _formattedDates = dates.map(formatter.parse).toList();
  
  //Apply sort function on formatted dates
  _formattedDates.sort();

  //Mapping through sorted dates to return a List<String> with same formatting as passed List's elements
  return _formattedDates.map(formatter.format).toList();

}

Solution 2

import 'package:intl/intl.dart';


 List dates = ["24-06-2021",
                  "27-05-2021",
                  "21-04-2021",
                  "29-07-2021",
                  "15-12-2021"];

  List<DateTime> newdates = [];
  DateFormat format = DateFormat("dd-MM-yyyy");

  for (int i = 0; i < dates.length; i++) {
    newdates.add(format.parse(dates[i]));
  }
  newdates.sort((a,b) => a.compareTo(b));

  for (int i = 0; i < dates.length; i++) {
    print(newdates[i]);
  }

Comments

[pskink]

an hour ago i gave you a solution with integers, the same works with DateTime objects

[Vinay Saurabh]

This code works, ` void main() { final List<String> dates=["2021-06-24","2021-05-24"]; var sorted = dates.map(DateTime.parse).toList()..sort((a,b) => a.compareTo(b)); for(int i =0;i<sorted.length;i++){ String date = '${sorted[i].day}-${sorted[i].month}-${sorted[i].year}'; print(date); } } ` but is it efficient. @pskink

[jamesdlin]

That's as efficient as you can get. You can't do a comparison-based sort faster than O(n log n), and converting everything to DateTime objects first means that you don't need to reparse the same strings when performing multiple comparisons of the same element.

[Alex Radzishevsky]

Date format needs to be fixed to dd-MM-yyyy

[Vinay Saurabh]

Thanks guys, have tweaked it a little bit and finally got my code.