Dart - find duplicate values on a List

Question

how can I find duplicate values on a list, Let's say I got a List like this:

List<Map<String, dynamic>> users = [
    { "name": 'John', 'age': 18 },
    { "name": 'Jane', 'age': 21 },
    { "name": 'Mary', 'age': 23 },
    { "name": 'Mary', 'age': 27 },
  ];

How I can iterate the list to know if there are users with the same name?

You can get your answer from this question(It shows how to delete duplicate elements from a list) -https://stackoverflow.com/questions/12030613/how-to-delete-duplicates-in-a-dart-list-list-distinct — Prathamesh, May 23 '20 at 14:49
Does this answer your question? [How to delete duplicates in a dart List? list.distinct()?](https://stackoverflow.com/questions/12030613/how-to-delete-duplicates-in-a-dart-list-list-distinct) — Lunedor, May 23 '20 at 14:55

camillo777 · Accepted Answer · 2022-12-12T09:53:39.160

11

A simple way would be this:

void main() {
  List<Map<String, dynamic>> users = [
    { "name": 'John', 'age': 18 },
    { "name": 'Jane', 'age': 21 },
    { "name": 'Mary', 'age': 23 },
    { "name": 'Mary', 'age': 27 },
  ];
  List names = []; // List();
  users.forEach((u){
    if (names.contains(u["name"])) print("duplicate ${u["name"]}");
    else names.add(u["name"]);
  });
}

Result:

duplicate Mary

edited Dec 12 '22 at 09:53

answered May 23 '20 at 15:19

camillo777

2,177
1
20
20

1

Change : List names = List(); ===> List names = []; – Rouatbi Daly Nov 29 '22 at 14:22

Rafael Ruiz Muñoz · Answer 2 · 2021-02-26T23:54:58.077

Probably a cleaner solution with extensions.

By declaring:

extension ListExtensions<E> on List<E> {
  List<E> removeAll(Iterable<E> allToRemove) {
    if (allToRemove == null) {
      return this;
    } else {
      allToRemove.forEach((element) {
        this.remove(element);
      });
      return this;
    }
  }

  List<E> getDupes() {
    List<E> dupes = List.from(this);
    dupes.removeAll(this.toSet().toList());
    return dupes;
  }
}

then you can find your duplicates by calling List.getDupes()

Note that the function removeAll doesn't exist in my current Dart library, in case you're reading this when they implement it somehow.

Also keep in mind the equals() function. In a List<String>, ["Rafa", "rafa"] doesn't contain duplicates.

If you indeed want to achieve this level of refinement, you'd have to apply a distinctBy function:

extension ListExtensions<E> on List<E> {
  List<E> removeAll(Iterable<E> allToRemove) {
    if (allToRemove == null) {
      return this;
    } else {
      allToRemove.forEach((element) {
        this.remove(element);
      });
      return this;
    }
  }

  List<E> distinctBy(predicate(E selector)) {
    HashSet set = HashSet();
    List<E> list = [];
    toList().forEach((e) {
      dynamic key = predicate(e);
      if (set.add(key)) {
        list.add(e);
      }
    });

    return list;
  }

  List<E> getDupes({E Function(E) distinctBy}) {
    List<E> dupes = List.from(this);
    if (distinctBy == null) {
      dupes.removeAll(this.toSet().toList());
    } else {
      dupes.removeAll(this.distinctBy(distinctBy).toSet().toList());
    }

    return dupes;
  }
}

kobowo · Answer 3 · 2021-04-14T13:48:44.297

I had a feeling Rafael's answer had code similar to Kotlin so I dug around and saw that these functions are part of the kt_dart library which basically gets the Kotlin standard library and ports it to Dart.

I come from a Kotlin background so I use this package often. If you use it, you can simply make the extension this much shorter:

extension KtListExtensions<T> on KtList<T> {
  KtList<T> get duplicates => toMutableList()..removeAll(toSet().toList());
}

just make sure to add kt_dart on your pubspec: kt_dart: ^0.8.0

Example

final list = ['apples', 'oranges', 'bananas', 'apples'].toImmutableList();
final duplicates = list.duplicates; // should be ['apples'] in the form of an ImmutableList<String>

score 1 · Answer 4 · answered May 31 '22 at 06:45

1

void main() {

  List<String> country = [
    "Nepal", 
    "Nepal", 
    "USA",
    "Canada",
    "Canada",
    "China",
    "Russia",
];
List DupCountry = [];
country.forEach((dup){
if(DupCountry.contains(dup)){
print("Duplicate in List= ${dup}");
}
else{
  DupCountry.add(dup);
}
});

}

answered May 31 '22 at 06:45

Imran Ali Khan

11
1

As it’s currently written, your answer is unclear. Please [edit] to add additional details that will help others understand how this addresses the question asked. You can find more information on how to write good answers [in the help center](/help/how-to-answer). – Community May 31 '22 at 08:19

score 1 · Answer 5 · answered Jun 19 '23 at 13:32

Here is my more concise and dependency free version to get all the duplicates or check for duplicates. The key is the toSet function that removes all the duplicates (we then compare the set and the list to identify if we have any)

extension List_E on List {
  bool hasDuplicates() => this.length != this.toSet().length;

  List getDuplicates() => this.where((x) => !this.toSet().remove(x)).toList();
}

Dart - find duplicate values on a List

5 Answers5