std: Iterator Adapter Catalog

This MOC links the standard-library iterator method-family notes for choosing the right adapter or consuming operation in Rust edition 2024 / stable 1.85+.

What it is

This catalog is a map for practical Iterator method selection. It groups related methods by the job they do. Each linked note is intentionally atomic at the method-family level. The focus is the stable standard library. Nightly-only methods mentioned in rustdoc are not presented as primary tools. Use this MOC when you know what you want a pipeline to do but not which method name best expresses it. Use Iterator Adapters for the broad concept of lazy iterator transformation. Use Consuming Adapters for terminal methods that consume an iterator and return a non-iterator result. Use The Iterator Trait when implementing custom iterators. Use Iterators for the language-level iteration model.

Method families

• Iterator map and filter - transform items, keep matching items, and combine both with filter_map.
• Iterator fold and reduce - consume many items into one accumulator result.
• Iterator flat_map and flatten - expand nested iterables and remove one nesting layer.
• Iterator zip and enumerate - pair streams together or attach indices.
• Iterator take skip and while bounds - keep or discard prefixes by count or predicate.
• Iterator chain cycle and step_by - concatenate, repeat, or sample traversal.
• Iterator collect and FromIterator - materialize iterators into destination types.
• Iterator scan and peekable - carry state or inspect one item of lookahead.
• Iterator partition and unzip - split one stream into two collections.
• Iterator sum product and count - compute common totals.
• Iterator predicate search adapters - short-circuit predicate questions and first-match searches.
• Iterator rev and last - reverse traversal or consume to the final yielded item.

Choosing quickly

• Need one output for each input: Iterator map and filter.
• Need to remove some items: Iterator map and filter.
• Need zero or one output from fallible parsing: Iterator map and filter.
• Need zero or more outputs per input: Iterator flat_map and flatten.
• Need a running total while still yielding values: Iterator scan and peekable.
• Need only a final total: Iterator fold and reduce.
• Need ordinary addition or multiplication: Iterator sum product and count.
• Need the number of yielded items: Iterator sum product and count.
• Need first match or first matching index: Iterator predicate search adapters.
• Need existence or validation: Iterator predicate search adapters.
• Need to pair two streams: Iterator zip and enumerate.
• Need to add current index: Iterator zip and enumerate.
• Need a prefix or page: Iterator take skip and while bounds.
• Need to concatenate two streams: Iterator chain cycle and step_by.
• Need to repeat a pattern: Iterator chain cycle and step_by.
• Need reverse traversal: Iterator rev and last.
• Need owned storage at the end: Iterator collect and FromIterator.

Stable scope

The source rustdoc includes nightly-only methods such as try_collect, collect_into, partition_in_place, and try_reduce. Those are deliberately not central notes in this catalog. For stable Rust edition 2024 / 1.85+, prefer stable methods listed in the linked notes. When a nightly method looks attractive, check whether stable collect, Extend::extend, try_fold, or an explicit loop expresses the same behavior. For third-party iterator helpers, cite docs.rs and verify the latest version before adding a note. This catalog currently uses only std.

Common composition paths

• Parse text values: Iterator map and filter → Iterator collect and FromIterator.
• Validate all inputs: Iterator predicate search adapters → Result.
• Build a summary: Iterator fold and reduce or Iterator sum product and count.
• Traverse nested data: Iterator flat_map and flatten → Iterator map and filter.
• Split data into groups: Iterator partition and unzip.
• Attach row numbers: Iterator zip and enumerate.
• Consume a header and keep parsing: Iterator take skip and while bounds with by_ref.
• Implement lightweight parser lookahead: Iterator scan and peekable.
• Cap infinite streams: Iterator take skip and while bounds before Iterator collect and FromIterator.
• Reverse a finite sequence: Iterator rev and last.

Pitfall index

• Unconsumed Iterator Adapters - lazy adapters do nothing until consumed.
• Unnecessary Collect - collecting too early wastes allocation and loses streaming.
• Needless Clone - cloning before filtering or taking can copy discarded data.
• Manual Index Loops for Speed - manual indexing is often less clear than enumerate.
• Unwrap and Expect Overuse - many search and reduction methods return Option.
• Integer Overflow - sums, products, counts, positions, and enumerate indices can overflow.
• Index Panics vs get - iterator traversal often avoids direct indexing panics.
• Returning References to Locals - iterator closures cannot return references to temporaries.

See also

Iterator Adapters · Consuming Adapters · The Iterator Trait · Iterators · Lazy Evaluation · Closures · Fn, FnMut, FnOnce · Option · Result · Vec · String and str · Type Inference

Sources

• Rust standard library, Iterator trait - std, https://doc.rust-lang.org/std/iter/trait.Iterator.html