Dart - How to sort Map's keys

Question

I have question in sorting Map's key in Dart.

Map<String, Object> map = new Map();

How can I sort the keys in map? or Sort the Iterable map.keys.

score 92 · Answer 1 · edited Jun 11 '20 at 02:58

92

In Dart, it's called SplayTreeMap:

import "dart:collection";

main() {
  final SplayTreeMap<String, Map<String,String>> st = 
      SplayTreeMap<String, Map<String,String>>();

  st["yyy"] = {"should be" : "3rd"};
  st["zzz"] = {"should be" : "last"};
  st["aaa"] = {"should be" : "first"};
  st["bbb"] = {"should be" : "2nd"};

  for (final String key in st.keys) {
    print("$key : ${st[key]}");
  }
}

// Output:
// aaa : first
// bbb : 2nd
// yyy : 3rd
// zzz : last

edited Jun 11 '20 at 02:58

Philippe Fanaro

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answered Aug 18 '13 at 10:57

Ilya Kharlamov

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error with ['should be'] : The method '[]' isn't defined for the class 'Object'. – Julien Jun 25 '19 at 11:37
@Julien Which version of dart that is? Used to work in dart 1.0 – Ilya Kharlamov Jun 26 '19 at 13:50
dart 2, the latest – Julien Jun 26 '19 at 13:51
It seems to work correctly on DartPad (https://dartpad.dev/dart) as of today. – Slick Slime Aug 12 '20 at 06:02
1

I don't understand the use case of this `SplayTreeMap` example. The original question was about sorting a `Map`'s keys; a `Map`'s values are random access, so getting a `List` of the keys and sorting it should be enough to achieve the same result. – Ganymede Mar 13 '21 at 08:07

Ganymede · Accepted Answer · 2016-03-15T03:11:59.120

54

If you want a sorted List of the map's keys:

var sortedKeys = map.keys.toList()..sort();

You can optionally pass a custom sort function to the List.sort method.

Finally, might I suggest using Map<String, dynamic> rather than Map<String, Object>?

edited Mar 15 '16 at 03:11

answered Aug 15 '13 at 01:24

Ganymede

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Watch out for one-liner `map.keys.toList().sort()`. In order to keep this in the same line, the cascade style should be used, as in `map.keys.toList()..sort()`. – nunobaba Dec 08 '13 at 20:23

score 8 · Answer 3 · answered Nov 09 '21 at 17:06

8

final sorted = SplayTreeMap<String,dynamic>.from(map, (a, b) => a.compareTo(b));

As seen at

How to use a SplayTreeMap on Firebase snapshot dictionary in dart/Flutter?

answered Nov 09 '21 at 17:06

BananaMaster

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score 6 · Answer 4 · answered Jul 09 '20 at 17:13

6

I know my answer is too late but check out what I've found.. Might help someone

This sortedmap Package helps to maintain Map of objects in a sorted manner.

answered Jul 09 '20 at 17:13

Jerin

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score 1 · Answer 5 · answered Mar 29 '22 at 10:26

1

map.entries.toList().sort(((a, b) => a.key.compareTo(b.key)));

I think so.

answered Mar 29 '22 at 10:26

Zero

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score 0 · Answer 6 · edited Jun 29 '23 at 16:14

I suggest the following code:

void main() {
  var x = {'b': 10, 'c': 7, 'a': 2};
  print('x = $x');
  
  final  y = x.orderByKeys(compareTo: (a, b) => a.compareTo(b));
  final  z = x.orderByValues(compareTo: (a, b) => a.compareTo(b));
  print('y = $y');
  print('z = $z');
  
  
}

/// Extensions on [Map] of <[K], [V]>
extension ExtendsionsOnMapDynamicDynamic<K,V> on Map<K, V>{
  /// Order by keys
  Map<K, V> orderByKeys({required int Function(K a, K b) compareTo}) {
    return Map.fromEntries(entries.toList()..sort((a, b) => compareTo(a.key, b.key)));
  }

  /// Order by values
  Map<K, V> orderByValues({required int Function(V a, V b) compareTo}) {
    return Map.fromEntries(entries.toList()..sort((a, b) => compareTo(a.value, b.value)));
  }
}

Dart - How to sort Map's keys

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