How deeply does The Well-Grounded Rubyist, Fourth Edition cover select!?

Explains block-based filtering and in-place behavior, with a hash-specific account of the nil return when nothing changes.

Select!

As explained in The Well-Grounded Rubyist, Fourth Edition

select! is a mutating collection method that removes elements failing a block-based selection test from the receiver and returns the modified collection or nil when no change occurs.

Use `select!` when a collection should keep only the elements that pass a block’s test—and the original collection should change. Arrays, hashes, and sets provide this in-place counterpart to `select` (also called `filter`). section 10.3.2

Keep selected hash pairs

`h = {1 => 2, 3 => 4, 5 => 6}` `h.select! {|key, value| key > 1 } # => {3 => 4, 5 => 6}` `h # => {3 => 4, 5 => 6}` `h.select! {|key, value| key > 1 } # => nil` The block tests each key/value pair. The first call removes a pair; the second has nothing left to remove, so it returns `nil`. section 9.3.5

The return value matters: `nil` means the selection made no change, not that it produced an empty collection. Unlike `select!`, `select` returns a separate result. For a hash operation that changes the receiver but returns that hash even when unchanged, use `keep_if`. section 9.3.5 section 10.3.2

`reject!` is the in-place counterpart for the opposite test: it removes elements for which the block returns true. There is also a `filter!` alias for `select!`, but no `find_all!` synonym. section 9.3.5 section 10.3.2

How The Well-Grounded Rubyist, Fourth Edition covers it

Cover of The Well-Grounded Rubyist, Fourth Edition

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Related concepts

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findcontrasts with · A find operation searches a collection for the first element or entry matching a value or predicate and returns it, or indicates that no match exists.
enumerable protocoluses · An enumerable protocol is an interface or abstraction that exposes elements or generated values for sequential traversal and generic processing.
lazy enumeratorused by · A lazy enumerator is a Ruby enumerator that defers producing elements until results are requested.
grepalternative to · grep is a Ruby Enumerable method that selects elements whose case-equality comparison with a supplied argument is true and can optionally transform the selected elements with 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.