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Dart: Converting a List to a Map and Vice Versa

Last updated on August 19, 2021 A Goodman Loading... Loading... Post a comment

Dart

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In this article, we will go over a couple of different ways to convert a list to a map as well as turn a map into a map in Dart. Without any further ado, lets dive right in.

Table of Contents

• Turning Lists into Maps
  • Using List.asMap[] method
  • Using Map.fromIterable or List.forEach method
• Converting Maps to Lists
  • Using Map.entries
  • Using Map.forEach method
• Conclusion

Turning Lists into Maps

Using List.asMap[] method

This method will return an immutable map from a given list. The returned map uses the indexes of the list as keys and the corresponding objects as values.Advertisements

Example:

// kindacode.com List list = ['orange', 'banana', 'apple', 'papaya']; void main[] { Map map = list.asMap[]; print[map]; print[map[0]]; print[map[1]]; }

Output:

{0: orange, 1: banana, 2: apple, 3: papaya} orange banana

If you try to modify the map, an error will occur. If you need to update the map without errors, see other approaches.

Using Map.fromIterable or List.forEach method

AdvertisementsThis approach is much more flexible than the above one. You can decide how the keys and values of the map look like.

Example

Lets say we have a list of users and we want to create a map from this list. This map will use the ids of the users as the keys.

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Using Map.fromIterable:

// kindacode.com List list = [ {'_id': 'u1', 'name': 'John Doe', 'age': 30}, {'_id': 'u2', 'name': 'Robbin', 'age': 40}, {'_id': 'u3', 'name': 'Tom', 'age': 50} ]; void main[] { Map map = Map.fromIterable[list, key: [item] => item['_id'], value: [item] => {'name': item['name'], 'age': item['age']}]; print[map]; }

Output:

{ u1: {name: John Doe, age: 30}, u2: {name: Robbin, age: 40}, u3: {name: Tom, age: 50} }

You can also use the List.forEach method to get the same results, as shown in the code snippet below:

// main.dart List list = [ {'_id': 'u1', 'name': 'John Doe', 'age': 30}, {'_id': 'u2', 'name': 'Robbin', 'age': 40}, {'_id': 'u3', 'name': 'Tom', 'age': 50} ]; void main[] { Map map = {}; list.forEach[[item] { map[item['_id']] = {'name': item['name'], 'age': item['age']}; }]; print[map]; }

Converting Maps to Lists

Dart supports several tools that can help us get the job done with ease.

Using Map.entries

Example:

// kindacode.com Map map = { 'key1': 'value1', 'key2': 'value2', 'key3': 'value3', 'key4': 'value4' }; void main[] { List list = []; // This list contains both keys and values List listValues = []; // This list only keeps values List listKeys = []; // This list only contains keys map.entries.map[[e] => list.add[{e.key: e.value}]].toList[]; print[list]; map.entries.map[[e] => listValues.add[e.value]].toList[]; print[listValues]; map.entries.map[[e] => listKeys.add[e.key]].toList[]; print[listKeys]; }

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Output:

[{key1: value1}, {key2: value2}, {key3: value3}, {key4: value4}] [value1, value2, value3, value4] [key1, key2, key3, key4]

Using Map.forEach method

Example:

// kindacode.com Map map = {'key1': 'value1', 'key2': 'value2', 'key3': 'value3'}; void main[] { List list = []; List keys = []; List values = []; map.forEach[[key, value] { // List of keys and values list.add[{key: value}]; // List of keys keys.add[key]; // List of values values.add[value]; }]; print[list]; print[keys]; print[values]; }

Output:

[{key1: value1}, {key2: value2}, {key3: value3}] [key1, key2, key3] [value1, value2, value3]

Conclusion

Weve walked through a few examples of converting a list to a map and a map to a list in Dart. If youd like to explore more new and interesting features of modern Dart and Flutter, take a look at the following articles:

• Dart: How to Add new Key/Value Pairs to a Map
• Using Cascade Notation in Dart and Flutter
• Dart: Convert Timestamp to DateTime and vice versa
• Dart & Flutter: 2 Ways to Count Words in a String
• Sorting Lists in Dart and Flutter [5 Examples]

You can also take a tour around our Flutter topic page and Dart topic page to see the latest tutorials and examples.

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