Dart/Flutter - Count the Occurrences of Elements in a List

This tutorial shows you how to count the frequency of elements in a List using Dart.

List is a data type that stores a collection of elements. Unlike Set, a List allows you to store duplicate elements. In other words, an element can appear multiple times in a List. If you need to find out how many times a value is in a List using Dart programming language, below are the examples.

This tutorial uses the below List for example.

  final values = ['a', 'b', 'a', 'c', 'd', 'b', 'e', 'd', 'a', 'c', 'b'];

Using Loop

The most basic way to count the frequency of an element is by iterating over the List. Create a variable as a counter and increment the counter if the current element matches the one to be counted.

  int countOccurrencesUsingLoop(List<String> values, String element) {
    if (values.isEmpty) {
      return 0;
    }

    int count = 0;
    for (String value in values) {
      if (value == element) {
        count++;
      }
    }

    return count;
  }

  countOccurrencesUsingLoop(values, 'a');
  // Output: 3

Using Iterable.where

Dart's List is a sub-type of Iterable and therefore it can use Iterable's methods. For this purpose, the most suitable method to use is where. It returns a new Iterable with all elements that satisfy the function passed as a predicate.

The passed function will be applied to each element. It accepts a parameter which is the element value and returns a bool value. An element is included in the new List if the function returns true. In this case, the function should return true only if the passed value matches the element to be counted. Then, access the length property to get the number of occurrences. The code is much shorter than the previous example.

  int countOccurrencesUsingWhere(List<String> values, String element) {
    return values.where((e) => e == element).length;
  }

Example:

  countOccurrencesUsingWhere(values, 'a');
  // Output: 3

Count the Occurrences of Each Element

If you have a List that's rarely modified and the system needs to get the frequency of any element many times, it's better to store the frequency of each element in a Map. The Map's keys are the elements, while the values are the occurrences of the respective element. Therefore, you only need one iteration to build the counter. Then, you can get the occurrence of any element from the Map.

  Map<String, int> countAllOccurrences(List<String> values) {
    final counter = <String, int>{};
    for (final value in values) {
      counter.update(value, (count) => count + 1, ifAbsent: () => 1);
    }

    return counter;
  }

Example:

  Map<String, int> counter = countAllOccurrences(values);
  // Output:  {a: 3, b: 3, c: 2, d: 2, e: 1}

  counter['a']; // 3

Summary

There are several ways to count the occurrences of elements in a List using Dart. The most basic way is using a loop. Another alternative with much shorter code is using the where method of Iterable. If there is a requirement to get the frequency of elements over and over, the recommended way is to create a counter using a Map.