How to get Map keys by values in Dart?

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?

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

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.

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.

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}');   
               } 
  
        }
          }

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

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

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'));
}

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)));

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;