Ruby: How to create a custom iteration method for a Enumerator Object - with enum_for?

Question

Lets say, we have an Array of Strings named words and I want to instantiate a special Enumerator using eunum_for on it which only counts Strings with the length != 3. Here is my proposal:

def words.n_each
  select{|x| x.length != 3}.each do |y|
    yield y
  end
end

e4 = words.enum_for(:n_each)
e4.each do |w|
  puts w
end

But there has to be a more smarter rubyish way to include the mechanics of the n_each method in enum_for - maybe inside a codeblock at the time of instantiation of the Enumerator. But how?

Your question is a bit unclear as to your desired outcome or usage. — engineersmnky
– engineersmnky, Commented Dec 10, 2021 at 17:09
based on my example, I am looking for e4 = words.enum_for(:select, {|x| x.length != 3} or something similar. — Eric
– Eric, Commented Dec 11, 2021 at 9:42

steenslag · Accepted Answer · 2021-12-09 20:45:58Z

2

Is lazy what you are looking for?

p e4 = ["aaa","c","foo","aaaa"].lazy.reject{|s| s.size == 3 } # => <Enumerator::Lazy: ["aaa", "c", "foo", "aaaa"]>:reject>

e4.each do |w|
  puts w
end

prints

c
aaaa

answered Dec 9, 2021 at 20:45

steenslag

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4 Comments

Eric Over a year ago

No I wasn't looking for lazy, but I was looking for this Syntax! You saved my evening. This is the way: e4 = words.enum_for.select{|x| x.length != 3} . Amazingly I have not found this syntax in any doc. Thanks!

engineersmnky Over a year ago

@Eric what do you mean not documented (Kernel#to_enum words.enum_for creates an each enumerator and then select returns an Array this would be identical to removing the enum_for in this case.

Eric Over a year ago

@engineersmnky: that's very clear, but in the case of each.meth the enumerator knows already what's to do. meth works on the return value of each (an enumerator). But in enum_for.meth meth is kind of an argument needed to create the enumerator. That's a little bit weird in my eyes.

Eric Over a year ago

I just figured out, that enum_for.meth does not create the custom enumerator. It behaves just like each.meth. That means, the syntax is consistent - but I am still looking for a possibility to create a custom enumerator with enum_for.

Cary Swoveland · Accepted Answer · 2021-12-10 00:12:45Z

0

class Array
  def each_except_size_3
    Enumerator.new do |y|
      (0..size-1).each do |i|
        s = self[i]
        y << s unless s.size == 3
      end
    end
  end
end

enum = ['guppy', 'owl', 'ox', 'cow', 'bear', 'lion'].each_except_size_3
  #=> #<Enumerator: #<Enumerator::Generator:0x00007fc1a41b9a28>:each>

enum.to_a
  #=> ["guppy", "ox", "bear", "lion"]

enum.take(4)
  #=> ["guppy", "ox", "bear", "lion"]

enum.take(3)
  #=> ["guppy", "ox", "bear"]

enum.take(99)
  #=> ["guppy", "ox", "bear", "lion"]

enum.next
  #=> "guppy"
enum.next
  #=> "ox"
enum.next
  #=> "bear"
enum.next
  #=> "lion"
enum.next
  #=> StopIteration: iteration reached an end

edited Dec 10, 2021 at 0:12

answered Dec 9, 2021 at 23:59

Cary Swoveland

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1 Comment

Eric Over a year ago

That's clear - my question aimed at the enum_for method.

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