Filtering Collections in Dart Using where

The where method in Dart is a powerful way to filter elements from collections such as lists and sets based on a specified condition. This method returns an iterable containing elements that satisfy the given predicate.

Filtering Lists

Example of Filtering with where

You can use the where method to filter a list based on specific criteria. Here’s an example:

void main() {
  List<int> numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

  // Filter even numbers   
  Iterable<int> evenNumbers = numbers.where((number) => number.isEven);
  
  print(evenNumbers.toList()); // Output: [2, 4, 6, 8, 10] }

Filtering with Multiple Conditions

You can also combine conditions in the where method:

void main() {
  List<int> numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

  // Filter numbers greater than 5 and even   Iterable<int> filteredNumbers = numbers.where((number) => number > 5 && number.isEven);
  
  print(filteredNumbers.toList()); // Output: [6, 8, 10] }

Filtering Sets

Example of Filtering with where

Similar to lists, you can filter sets using the where method:

void main() {
  Set<int> numbers = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};

  // Filter odd numbers   Iterable<int> oddNumbers = numbers.where((number) => number.isOdd);
  
  print(oddNumbers.toSet()); // Output: {1, 3, 5, 7, 9} }

Filtering Maps
 

Filtering Keys or Values in Maps

You can filter the keys or values of a map using the where method on the map's keys or values.

Example of Filtering Map Values

void main() {
  Map<String, int> ages = {
    'Alice': 30,
    'Bob': 25,
    'Charlie': 35,
    'David': 28,
  };

  // Filter names with age greater than 28   
  Iterable<String> filteredNames = ages.keys.where((name) => ages[name]! > 28);
  
  print(filteredNames.toList()); // Output: [Alice, Charlie, David] }

Example of Filtering Map Entries

You can also filter map entries based on both the key and value:

void main() {
  Map<String, int> ages = {
    'Alice': 30,
    'Bob': 25,
    'Charlie': 35,
    'David': 28,
  };

  // Filter entries where age is greater than 28   
  Iterable<MapEntry<String, int>> filteredEntries = ages.entries.where((entry) => entry.value > 28);
  
  // Convert filtered entries back to a map   
  Map<String, int> filteredMap = Map.fromEntries(filteredEntries);
  
  print(filteredMap); // Output: {Alice: 30, Charlie: 35, David: 28} 
  }

Conclusion

The where method in Dart is a versatile tool for filtering collections such as lists, sets, and maps. By providing a predicate function, you can easily extract elements that meet specific conditions, facilitating effective data manipulation and analysis. Mastering the use of where will enhance your coding efficiency when working with collections in Dart.