How to get Map keys by values in Dart?

Question

In Dart language how to get MAP keys by values?

I have a Map like;

{
  "01": "USD",
  "17": "GBP",
  "33": "EUR"
}

And I need to use values to get keys. How do I do that?

score 121 · Accepted Answer · answered Aug 28 '18 at 07:25

121

var usdKey = curr.keys.firstWhere(
    (k) => curr[k] == 'USD', orElse: () => null);

answered Aug 28 '18 at 07:25

Günter Zöchbauer

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216
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1,567

Thanks Günter its working... By the way where I can get details information for this? So I can study to understand and I can use late in my advance Map project? – Nick Aug 28 '18 at 07:30
3

I don't think this is documented as a thing by itself. There is the `keys` property of `Map` and the `firstWhere` method of `Iterable` and the rest is using them together in a meaningful way. https://api.dartlang.org/stable/2.0.0/dart-core/Map/keys.html, https://api.dartlang.org/stable/2.0.0/dart-core/Iterable/firstWhere.html – Günter Zöchbauer Aug 28 '18 at 07:32

Randal Schwartz · Answer 2 · 2018-08-28T14:59:16.100

If you will be doing this more than a few times on the same data, you should create an inverse map so that you can perform simple key lookup instead of repeated linear searches. Here's an example of reversing a map (there may be easier ways to do this):

main() {
  var orig = {"01": "USD", "17": "GBP", "33": "EUR"};
  var reversed = Map.fromEntries(orig.entries.map((e) => MapEntry(e.value, e.key)));
  for (var kv in reversed.entries) {
    print(kv);
  }
}

Edit: yes, reversing a map can simply be:

var reversed = orig.map((k, v) => MapEntry(v, k));

Tip of the hat to Joe Conway on gitter. Thanks.

Makdir · Answer 3 · 2020-05-30T10:19:04.660

8

There is another one method (similar to Günter Zöchbauer answer):

void main() {

  Map currencies = {
     "01": "USD",
     "17": "GBP",
     "33": "EUR"
  };

  MapEntry entry = currencies.entries.firstWhere((element) => element.value=='GBP', orElse: () => null);

  if(entry != null){
    print('key = ${entry.key}');
    print('value = ${entry.value}');
  }

}

In this code, you get MapEntry, which contains key and value, instead only key in a separate variable. It can be useful in some code.

edited May 30 '20 at 10:19

answered May 30 '20 at 10:12

Makdir

390
2
8

This was useful since (element) contains both key & value and helped me in the test condition with both key & value, since I had same values but different keys but I required both entries. Since 'firstWhere' was ignoring the duplicate. Thanks – Ant D Aug 24 '20 at 06:03
1

Glad my answer helped you. MapEntry really help to visualize the structure of data, which can be very useful for debugging and testing. – Makdir Oct 30 '20 at 09:54
Thanks! Worked for me! This answer should have more upvote! – Al Mamun Mar 12 '22 at 02:30

score 6 · Answer 4 · edited Jun 12 '20 at 18:51

6

Map map = {1: 'one', 2: 'two', 3: 'three'};

var key = map.keys.firstWhere((k) => map[k] == 'two', orElse: () => null);
print(key);

edited Jun 12 '20 at 18:51

Matt Ke

3,599
12
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answered Jun 12 '20 at 16:44

Frank Gue

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3

While this code may help with the question, it is better to include some context, explaining how it works and when to use it. Code-only answers tend to be less useful in the long run. See [How do I write a good answer?](https://stackoverflow.com/help/how-to-answer) for some more info. – Klaus Gütter Jun 13 '20 at 15:52
in my case worked: map.entries.firstWhere((element) => element.value == 'two', orElse: () => null).key; – SardorbekR Feb 03 '21 at 11:24

Puneet Nagar · Answer 5 · 2019-08-04T06:53:34.373

2

You can do the following :

var mapper = { 
              '01' : 'USD',
               '17' : 'GBP'     } 

for(var val in mapper.keys){

  switch(mapper[val]){

        case 'USD' : {
                             print('key for ${mapper[val]} is : ' '${val}');  
            }

          break;

        case 'GBP' : {
                             print('key for ${mapper[val]} is : ' '${val}');   
               } 

        }
          }

edited Aug 04 '19 at 06:53

answered Aug 04 '19 at 06:46

Puneet Nagar

29
4

1

Not effective and reusable. You hardcode values in `switch`, which make it hard to maintain and refactor. – Michał Dobi Dobrzański Sep 02 '20 at 10:22

score 2 · Answer 6 · answered Sep 01 '20 at 18:32

If someone still need a solution, I wrote a simple library to deeply (search inside nested maps either) search by value inside Map. Usage is simple, because deepSearchByValue() is an extension method, so all you need to do is to import my library and call the method on your map:

import 'package:deep_collection/deep_collection.dart';


void main() {
  print({
    "01": "USD",
    "17": "GBP",
    "33": "EUR",
  }.deepSearchByValue((value) => value == 'USD'));
}

score 1 · Answer 7 · answered Jul 23 '21 at 23:54

1

Building on Randal Schwartz's answer:

Map<T, R> invertMap<T, R>(Map<R, T> toInvert) =>
  Map.fromEntries(toInvert.entries.map((e) => MapEntry(e.value, e.key)));

answered Jul 23 '21 at 23:54

Matthew Fala

31
1

score 1 · Answer 8 · answered Nov 05 '21 at 15:09

An extension of Randal Schwartz's answer.

extension InvertMap<K, V> on Map<K, V> {
  Map<V, K> get inverse => Map.fromEntries(entries.map((e) => MapEntry(e.value, e.key)));
}

This provides us with a highly reusable solution:

var inverseMap = originalMap.inverse;

How to get Map keys by values in Dart?

8 Answers8

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