Iterator

An iterator is an object that enables a programmer to traverse a sequence of elements, one at a time, without exposing the underlying structure. In common usage, an iterable is a collection or generator that can produce an iterator. An iterator maintains internal state about the current position and advances to the next element upon request.

Most languages separate the notions of iterable and iterator. An iterator exposes a method to return the next element and to detect completion. When no more elements exist, iteration ends. In Python, an iterator implements __iter__ (returning self) and __next__ (raising StopIteration). In Java, the Iterator interface provides hasNext and next; Java 8 adds forEachRemaining. In C#, IEnumerator provides MoveNext and Current, and IEnumerable enables obtaining an enumerator.

Iterators enable lazy evaluation: elements are produced on demand, which makes them suitable for large or infinite sequences. They support memory-efficient processing and can be composed in pipelines via map, filter, and reduce operations.

Common considerations: most iterators are single-pass; restarting requires acquiring a new iterator. Iteration may have side effects or depend on external state. Some iterators may raise exceptions when exhausted or when the source changes during iteration.

Applications include traversing lists, files, streams, or any data source that can be consumed sequentially. Iterators underpin language constructs such as for loops and comprehensions, and enable patterns like generator functions and lazy pipelines.