Many methods in Ruby array return an enumerator when invoked without parameters or blocks (index, keep_if, each, drop_while and many more).
- When is it appropriate to use methods in this form, as opposed to calling them with a block?
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2 Answers
From the docs to Enumerator:
Most methods have two forms: a block form where the contents are evaluated for each item in the enumeration, and a non-block form which returns a new Enumerator wrapping the iteration.
This allows you to chain Enumerators together. For example, you can map a list’s elements to strings containing the index and the element as a string via:
puts %w[foo bar baz].map.with_index {|w,i| "#{i}:#{w}" } # => ["0:foo", "1:bar", "2:baz"]An Enumerator can also be used as an external iterator. For example, Enumerator#next returns the next value of the iterator or raises StopIteration if the Enumerator is at the end.
e = [1,2,3].each # returns an enumerator object. puts e.next # => 1 puts e.next # => 2 puts e.next # => 3 puts e.next # raises StopIteration
I'm sorry for copy-paste, but I couldn't explain better.
Comments
The main original reason for the Enumerator class to exist is method chaining:
array.each.with_object [[], []] { |element, memo| ... }
So basically, you don't need to worry about that.
