Iterable

Learn iterable collections with Iterable.

Iterable<T>

Iterable<T> is the top collection interface. It is the common ancestor of List, Set and Queue. An Iterable is a sequence whose elements can be traversed one by one; but it does not support index access.

Every List or Set is also an Iterable. Methods like where, map and fold work through Iterable. These methods are lazy; the result is produced only when needed.

Example: Iterable<int> numbers = [1, 2, 3, 4, 5]; var evens = numbers.where((n) => n.isEven);. Here evens is not computed yet; it is produced when toList() is called.

Lazy evaluation

When you call where or map on an Iterable the result is not computed at once. Elements are produced only when an operation like toList(), length or a loop runs.

This is called lazy evaluation. The advantage is that no unnecessary computation happens. For example if you want only the first three even numbers from a large list the whole list is not filtered.

The toList() method turns the lazy result into a concrete list. This way you can print the result or access it by index.

Exercise Filter with Iterable

Define Iterable<int> numbers = [1, 2, 3, 4, 5]. Filter even numbers with where. Convert the result to a list with toList() and print it. Then print evens.length.

Show example solution

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

Test yourself

Define an Iterable

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

Show answer

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

Which property does Iterable have?
- Lazy and does not support indexing
- Holds only numbers
- Supports index access
- Has a fixed size
Show answer

Lazy and does not support indexing — Iterable is lazy and does not support index access.
Match the Iterable methods

where(...)

Filter
map(...)

Transform
toList()

Convert to list
fold(...)

Combine

Next lesson: Summary - Collections

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