在Ruby数组中使用返回枚举器的方法的正确方法是什么？

Question

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?

Answer 1

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.

Answer 2

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.