where
- Dart course
- Intermediate
- Lesson 48/116
- 5 min
Learn filtering with .where.
The where method
The where method picks elements that match a condition. The original list does not change; the result is a new Iterable. It is used for filtering.
Example: var evens = [1, 2, 3, 4].where((n) => n.isEven);. Here the condition n.isEven keeps only even numbers. The result is [2, 4].
Like map, where is lazy. To see the result you use toList(). The condition function keeps elements that return true and drops those that return false.
Filtering with where
The where method takes an arrow function that returns a bool. If the function returns true the element stays; if false it is dropped.
Example: var short = ['cat', 'dog', 'rabbit'].where((s) => s.length < 5);. Here fruits shorter than five letters are picked. Result: ['cat', 'dog'].
Another example: var positive = [-2, 0, 3].where((n) => n > 0);. Only positive numbers remain: [3]. where is the core tool for narrowing data by a condition.
Practice Filter even numbers
Call where on the list [1, 2, 3, 4, 5, 6]. The condition should be (n) => n.isEven. Convert the result to a list with toList() and print it.
Starter code
void main() {
var numbers = [1, 2, 3, 4, 5, 6];
// use .where to filter even numbers, convert with toList and print
}Sample solution
void main() {
var numbers = [1, 2, 3, 4, 5, 6];
var evens = numbers.where((n) => n.isEven);
print(evens.toList());
}Expected output
[2, 4, 6]
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What happens to an element when where returns false?
var a = [1, 2, 3].where((n) => n > 5);- It is added to the list
- It throws an error
- It is moved to the end
- It is dropped, not in the result Correct answer
Explanation: where drops elements that return false.
2 The where call
var numbers = [1, 2, 3, 4, 5, 6];
var evens = numbers.____((n) => n.isEven);var numbers = [1, 2, 3, 4, 5, 6];
var evens = numbers.where((n) => n.isEven);Explanation: where filters by a condition.
3 Match the where parts
| Term | Meaning |
|---|---|
(n) => n.isEven |
Even number condition |
.where(...) |
Filter call |
true |
Keep element |
false |
Drop element |