First element
A first operation retrieves the initial element of a sequence or collection, optionally subject to a predicate, and indicates when no matching element exists.
Clojure: The Essential Reference
first is a Clojure function that returns the first element of a sequenceable collection or nil when none is available.
- How it is used
- Selects a collection’s head during sequential processing, including transformations that extract one field from each item.
- How deep it goes
- Defines the one-argument contract and nil edge cases, then illustrates collection-dependent ordering and access alternatives without implementing the function.
- Examples
- Uses a list lookup, area-code extraction from phone numbers, and map and set lookups to show differing sequential views.
- What you can do after
- Apply it via Clojure’s standard library to retrieve a sequence head or extract leading fields with map.
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The Well-Grounded Rubyist, Fourth Edition
first is an Enumerable method that returns the first element or a specified number of initial elements from an enumerable object.
- How it is used
- Retrieves the beginning of arrays, ranges, hashes, and other Enumerable objects during collection querying.
- How deep it goes
- Explains return shapes, hash iteration, infinite enumerables, and lazy evaluation, but does not exhaustively catalog Enumerable methods.
- Examples
- Uses arrays, ranges, hashes, an infinite die iterator, and multiples of three selected from an infinite range.
- What you can do after
- Apply it via Ruby's Enumerable API to obtain one item or a finite initial slice from an iterable, including lazy infinite sequences.
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Related concepts
- lastcontrasts with · Last is an operation that returns or emits the final element encountered in a finite ordered collection, iterable, stream, or sequence.
- terminal operationkind of · A terminal operation ends a stream or sequence pipeline, triggers evaluation of preceding transformations, and produces a final result or side effect.
- query methodkind of · A query method is a method that defines, constructs, refines, or evaluates a database query.
- enumerable protocolpart of · An enumerable protocol is an interface or abstraction that exposes elements or generated values for sequential traversal and generic processing.
- sequenceuses · A sequence is an ordered collection of elements whose positions determine their order of traversal or processing.
- each_entryuses · each_entry is a Ruby enumeration method that yields each element of a collection to a block.
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