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Ruby Bits: Each with object

TheEnumerable module is the core of everything in Ruby. It is often said that if you know this module, then you know Ruby and that’s not far from the truth, in my opinion.

From all the awesome methods in enumerable there is one that I love above all, to the point that I have publicly admitted to be my favourite method in the entire language, and that is each_with_object.

Here’s how you could use it:

numbers = [1, 2, 3, 4]

numbers.each_with_object([]) do |number, acc|
  acc << number * 3
end

## => [3, 6, 9, 12]

It works be iterating over each of the elements of the Array, and performing some action on them, just like each, but with the difference that it also gives you an accumulator or memo object which I have called acc in the example above.

This object allows you to keep data from one step of the iteration to the other. If you’ve ever used inject or reduce it’s pretty much the same thing, but you don’t have to return the memo object at the end of the block, it is automatically passed on with whichever changes you’ve made to it.

The caveat with this method is that the memo object really does need to be an object, as it relies on the fact that it is passed by reference, not by value as Fixnums or Strings.

In The Real World™ this could be used to build up an Array of objects, for example:

  (1..10).each_with_object([]) do |num, questions|
    questions << Question(name: num.to_s)
  end

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Luis Zamith

About Luis Zamith

Has worked on the web for a while now, mainly using Ruby and Ruby on Rails. Enjoys open source and giving back to the community, having taught Rails to hundreds of people.
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