Dart: Remove Duplicate Element From a List

In the world of software development, particularly when working with Dart for Flutter applications, lists are a fundamental data structure. They store collections of items, such as user inputs, product details, or any dataset.

However, a common challenge arises when these lists contain duplicate elements. Duplicates can lead to inefficiencies, increased memory usage, and incorrect data processing, which are detrimental in both simple and complex applications.

Therefore, understanding how to effectively remove these duplicates is crucial for maintaining optimal performance and data integrity in your Dart applications.

The purpose of this discussion is to guide you, whether you’re a beginner or an experienced developer, through the process of identifying and removing duplicate elements from lists in Dart.

By the end of this guide, you’ll have a clear understanding of the different methods to achieve this, along with their respective pros and cons, enabling you to write more efficient and error-free code.

Understanding Lists in Dart

Before diving into the removal of duplicates, let’s quickly review what lists are in Dart and the types that are most relevant to our discussion on duplicate removal:

  • Basic Concept of a Dart List: A list in Dart is an ordered collection of items. It’s similar to arrays in other programming languages. Lists can store integers, strings, objects, or any other type of data.
  • Types of Lists:
    • Fixed-Length List: As the name suggests, the length of these lists cannot change after their creation. Attempting to add or remove items will result in an error.
    • Growable List: These lists can change in size. You can add or remove items dynamically, which makes them more flexible and commonly used in various applications.
  • Relevance to Duplicate Removal: When dealing with duplicate removal, we mostly work with growable lists. This is because they offer the flexibility needed to add or remove elements on the fly. Fixed-length lists are less commonly used in scenarios where duplicate removal is needed, as their size cannot be dynamically adjusted.

Identifying Duplicate Elements

In Dart, as in most programming languages, a duplicate element in a list refers to an item that appears more than once. Recognizing duplicates is crucial for data accuracy and performance optimization. Here’s a deeper look into what constitutes a duplicate in a Dart list and the common scenarios where they might occur:

What Constitutes a Duplicate in a Dart List?

  1. Basic Duplicate: The most straightforward case of a duplicate is when primitive data types like integers or strings are repeated. For example, in a list [1, 2, 2, 3], the number 2 is a duplicate.
  2. Object Duplication: If the list contains objects, duplication becomes a bit more complex. Two objects are considered duplicates if all their properties are identical. However, this can vary based on how the equality of objects is defined in your Dart code.
  3. Custom Criteria for Duplication: Sometimes, duplicates are defined based on specific criteria, like if only a particular attribute of an object is repeated. For example, in a list of Person objects, you might consider two persons as duplicates if they have the same email address, regardless of other properties.

Common Scenarios Where Duplicates Might Occur

  1. User Input: When receiving data from user input, there’s always a possibility of duplication, either accidentally or intentionally.
  2. Data Merging: When merging data from multiple sources, duplicates can arise. For example, combining two lists of email subscribers can result in repeated entries.
  3. Data Generation: In scenarios where data is programmatically generated or fetched from APIs, there can be repetitions due to overlaps or errors in the generation process.
  4. Complex Data Structures: When dealing with complex nested lists or objects within lists, it’s common to encounter duplicates, particularly if there’s a lack of proper checks during the data construction phase.

Techniques to Remove Duplicates

Removing duplicates from a list in Dart can be achieved through various methods, each with its own advantages. Below are three effective techniques:

Method 1: Using Set

A Set in Dart is a collection of unique items, which makes it a powerful tool for removing duplicates from a list.

Step-by-Step Guide:

Create a Set from the List: Convert your list into a Set. This process automatically removes any duplicate items since a Set only holds unique elements.

List<int> myList = [1, 2, 2, 3, 3, 4];
Set<int> mySet = Set.from(myList);

Convert Back to List: If you need the result in a list format, simply convert the Set back to a list.

List<int> uniqueList = mySet.toList();

Result: The uniqueList will now have only unique elements: [1, 2, 3, 4].

Method 2: Iterative Approach

This approach involves manually checking each item in the list and building a new list with only unique items.

Detailed Walkthrough:

Initialize a New List: Create an empty list that will hold the unique elements.

List<int> uniqueList = [];

Iterate and Check: Loop through the original list. For each element, check if it is already in the unique list. If not, add it.

for (int item in myList) {
  if (!uniqueList.contains(item)) {

Result: The uniqueList now contains unique elements from the original list.

Method 3: Dart’s Built-in Functions

Dart provides built-in functions like .toSet() and .distinct() to streamline the process of removing duplicates.

Utilizing .toSet() and .distinct():

Using .toSet(): As seen in Method 1, .toSet() can be used to remove duplicates and then convert back to a list.

List<int> uniqueList = myList.toSet().toList();

Using .distinct(): The .distinct() method returns an iterable that provides unique elements from the list.

List<int> uniqueList = myList.distinct().toList();
List<int> uniqueList = myList.distinct().toList();

Result: Both methods will yield a list without duplicate elements.

Comparative Analysis of Methods for Removing Duplicates in Dart

When choosing a method to remove duplicates from a list in Dart, it’s important to consider factors like performance, memory usage, and code maintainability. Here’s a comparative analysis of the three methods discussed:

1. Using Set

  • Performance: Converting a list to a set and back is generally efficient, especially for large lists. The process of removing duplicates happens internally and is optimized in Dart.
  • Memory Usage: This method temporarily increases memory usage since it involves creating a new set. However, the impact is minimal and is usually offset by the efficiency of the operation.
  • Code Maintainability: The code is concise and easy to understand. It leverages Dart’s built-in features, making it a maintainable and idiomatic approach.

2. Iterative Approach

  • Performance: This method can be less efficient compared to using a set, especially for large lists, as it involves manually checking each element with a loop.
  • Memory Usage: Memory usage is similar to the Set method, as it also creates a new list. However, the iterative checks can add a bit of overhead.
  • Code Maintainability: While this method provides more control, it results in more verbose code. It can be less maintainable, especially if not implemented carefully.

3. Dart’s Built-in Functions (toSet() and distinct())

  • Performance: Utilizing built-in functions like toSet() and distinct() is efficient and similar in performance to manually using a set. These methods are optimized internally by Dart.
  • Memory Usage: Memory usage patterns are similar to using a Set explicitly. The functions handle the conversion and unique element storage internally.
  • Code Maintainability: These methods provide the most maintainable and concise code. They are part of Dart’s standard library, making the code idiomatic and easy to read.

Handling Complex Cases

Removing duplicates in lists can become more complex when dealing with custom objects or large datasets. Here’s how to approach these scenarios:

Removing Duplicates in Lists of Custom Objects

  1. Define Equality for Objects: Dart does not automatically know how to compare custom objects for equality. You need to define what makes two objects ‘equal’. This could be based on one or multiple properties of the objects.
  2. Overriding equals and hashCode: Implement the equals method and hashCode for your objects. This ensures that sets and maps can correctly identify duplicates.
  3. Using a Custom Function with distinct(): Dart’s distinct() function can take a custom function that defines equality. This is useful for lists of custom objects.
class Person {
  String name;
  String email;
  // Constructor, equals, and hashCode methods...

List<Person> uniquePersons = personsList.distinct((p) => p.email).toList();

Strategies for Large Lists and Performance Considerations

  1. Efficiency with Sets: For very large lists, converting to a set and back to a list is often the most efficient method, as it leverages Dart’s optimized internal handling of sets.
  2. Lazy Iterables: When using methods like distinct(), consider working with lazy iterables (not converting to a list immediately) if you’re not going to need all elements at once. This can save memory.
  3. Chunk Processing: In cases of extremely large datasets, consider processing the list in chunks to avoid large memory spikes.

Best Practices and Common Pitfalls

Best Practices

  1. Use Built-in Functions: Leverage Dart’s built-in functions for simplicity and efficiency.
  2. Define Equality Clearly: In custom objects, clearly define what constitutes equality to avoid unexpected behavior.
  3. Test for Edge Cases: Especially with custom equality, test your implementation with various edge cases.

Common Pitfalls

  1. Ignoring Object Equality: Not properly overriding equals and hashCode in custom objects can lead to incorrect duplicate removal.
  2. Memory Overuse: Be cautious with very large lists; using inefficient methods can lead to high memory usage and slow performance.
  3. Modifying Original List: Avoid modifying the original list unless absolutely necessary. Working with a copy of the list is safer to prevent unintended side effects.


To recap, removing duplicates from lists in Dart can be handled efficiently using a variety of methods:

  • Using Sets (manually or with toSet()) is a robust default choice for its efficiency and simplicity.
  • Iterative approaches offer more control and are suitable for complex scenarios but can be less efficient.
  • Built-in functions (distinct()) are particularly useful for custom object comparisons and provide a balance between performance and readability.
Hussain Humdani

Hussain Humdani

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