How deeply does The Well-Grounded Rubyist, Fourth Edition cover first element?

Explains return shapes, hash iteration, infinite enumerables, and lazy evaluation, but does not exhaustively catalog Enumerable methods.

First element

As explained in 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.

first retrieves the first item produced by an enumerable. With an array or range, calling first without an argument returns one object; passing n returns an array containing the first n objects, even when n is 1. section 9.2.5 section 10.4.1

The returned value follows the enumerable's iteration behavior:

An array returns its initial element, while a range returns the first value it yields. section 10.4.1
A hash returns its first inserted key/value pair as a two-element array. section 10.4.1
The method finds the beginning of an iteration, unlike last, which has no general meaning for an enumerable that can continue forever. section 10.4.1

Taking a finite prefix from an infinite sequence

A finite range can be filtered and then queried directly, but an infinite range must be lazy: `(1..Float::INFINITY).lazy.select { |n| n % 3 == 0 }.first(10)` returns `[3, 6, 9, 12, 15, 18, 21, 24, 27, 30]`. The lazy pipeline evaluates only enough values to produce the requested ten results. Alternatively, `take(n).force` converts a selected lazy prefix into an ordinary result array. section 10.12

Table 9.2 Summary of common array query methods

Method name/sample call	Meaning
`a.size (synonyms: length, count)`	Number of elements in the array
`a.empty?`	True if `a` is an empty array; false if it has any elements
`a.include?(item)`	True if the array includes `item`; false, otherwise
`a.count(item)`	Number of occurrences of `item` in array
`a.first(n=1)`	First n elements of array
`a.last(n=1)`	Last n elements of array
`a.sample(n=1)`	n random elements from array

Compare first with size, empty?, include?, count, last, and sample to see how array queries differ in purpose and return shape. section 9.2.5

How The Well-Grounded Rubyist, Fourth Edition covers it

Cover of The Well-Grounded Rubyist, Fourth Edition

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Clojure: The Essential Referencefirst is a Clojure function that returns the first element of a sequenceable collection or nil when none is available.

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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.

This explainer was generated by Manning's knowledge graph from the text of The Well-Grounded Rubyist, Fourth Edition. The section links point to the parts of the book it draws on.