How to convert a List into a Map in Dart

Question

I looking for an on-the-shelf way to convert a List into a Map in Dart.

In python for example you can do:

l= [ ('a',(1,2)), ('b',(2,3)), ('c',(3,4) ) ]
d=dict(l)
==> {'a': (1, 2), 'c': (3, 4), 'b': (2, 3)}

The dict function expects a List of couple. For each couple, the first element is used as the key and the second as the data.

In Dart I saw the following method for a List : asMap(), but it's not doing what i expect: it use the list index as key. My questions:

• Do you known anything in Dart libraries to do this ?
• If not, any plan to add such a feature in the core lib ?

Proposal:

List.toMap() //same as python dict.
List.toMap( (value) => [ value[0], value[1] ] ) //Using anonymous function to return a key and a value from a list item.

Thanks and Regards,

Nicolas

Answer 1

You can use Map.fromIterable:

var result = Map.fromIterable(l, key: (v) => v[0], value: (v) => v[1]);

or collection-for (starting from Dart 2.3):

var result = { for (var v in l) v[0]: v[1] };

Answer 2

In Dart 1.1, you can use this constructor of Map:

new Map.fromIterable(list, key: (v) => v[0], value: (v) => v[1]);

This would be closest to your original proposal.

Answer 3

For Dart 2.3 and more ,

There are many solutions, but the simplest are :

1- Use the asMap() function

List list = ['steve', 'bill', 'musk'];
Map map = list.asMap();
print(map);

Output :

{0: steve, 1: bill, 2: musk}

2- Use for-in collection inside your map Map map = {};

List list = ['steve', 'bill', 'musk'];
Map map = {for (var item in list) '$item' : 'valueOf$item'};
print(map);

Output :

{steve: valueOfsteve, bill: valueOfbill, musk: valueOfmusk}

Hope it will be helpful .

Answer 4

Another possibility is to use the Map.fromEntries() constructor:

final result = Map.fromEntries(l.map((value) => MapEntry(value[0], value[1])));

Answer 5

Details

• Flutter 1.26.0-18.0.pre.106

Solution

/libs/extensions/list.dart

extension MapFromList<Element> on List<Element> {
  Map<Key, Element> toMap<Key>(
          MapEntry<Key, Element> Function(Element e) getEntry) =>
      Map.fromEntries(map(getEntry));

  Map<Key, Element> toMapWhereKey<Key>(Key Function(Element e) getKey) =>
      Map.fromEntries(map((e) => MapEntry(getKey(e), e)));
}

Usage

import 'package:myApp/libs/extensions/list.dart';

final list = [1,2,3,4];
print(list.toMap((e) => MapEntry(e-1, e)));  // {0: 1, 1: 2, 2: 3, 3: 4}
print(list.toMapWhereKey((e) => "str $e"));  // {str 1: 1, str 2: 2, str 3: 3, str 4: 4}

Answer 6

This may not directly relevant to the answer but, if there is anyone who wants to convert.

var list = ['fee','fi','fo','fum'];

to a Map like,

var map = {0:'fee', 1:'fi', 2:'fo', 3:'fum'};

you can use

var map = list.asMap();

PS

This map is not editable

Answer 7

There is not yet a concept of tuples or pairs in dart (other than a two element Iterable). If Dart had such a concept, then issue 7088 proposes to solve this with a Map.fromPairs constructor.