Iterators

An iterator is an object that yields elements of a sequence one at a time. An iterable is an object that can produce an iterator. The iterator maintains internal state and, on each request, returns the next element until the sequence is exhausted. Once exhausted, subsequent requests are handled differently depending on the language, such as signaling termination or raising a specific exception.

In many languages, the interface and conventions differ. Python, for example, uses an iterator protocol where an object provides a __iter__ method to return an iterator and a __next__ method to produce the next value, raising StopIteration when the end is reached. Java exposes an Iterator interface with hasNext and next methods. C++ relies on iterator objects that act like pointers, advancing with increment operations and dereferencing to access elements. Rust defines an Iterator trait with a next method that returns an optional value. Despite syntax differences, the core idea is the same: a stateful provider that advances through elements on demand.

Key features of iterators include lazy evaluation, meaning elements are produced only as needed, and memory efficiency, which enables working with large or infinite sequences. Iterators are composable through higher-order operations such as map, filter, and zip, enabling pipelines for data processing without materializing intermediate results.

Common caveats include that many iterators are single-pass: once exhausted, they may not be reset without recreating the underlying iterable. Understanding the distinction between iterables and iterators helps in writing efficient, reusable data-processing code.