where

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.

Exercise 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.

Show example solution

void main() {
  var numbers = [1, 2, 3, 4, 5, 6];
  var evens = numbers.where((n) => n.isEven);
  print(evens.toList());
}

Test yourself

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
Show answer

It is dropped, not in the result — where drops elements that return false.

The where call

var numbers = [1, 2, 3, 4, 5, 6];
var evens = numbers.____((n) => n.isEven);

Show answer

var numbers = [1, 2, 3, 4, 5, 6];
var evens = numbers.where((n) => n.isEven);

Match the where parts

(n) => n.isEven

Even number condition
.where(...)

Filter call
true

Keep element
false

Drop element

Next lesson: fold and reduce

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies