Filling a ruby array with a default object

Question

Let's say I have a class Counter in ruby, defined as

class Counter
    attr_accessor :starting_value
    def initialize(starting_value)
        @starting_value = starting_value
    end

    def tick
        @starting_value = @starting_value + 1
    end
end

and I want to fill an array with that object, using a default parameter, like this: counter_arr = Array.new(5, Counter.new(0))

This is almost what I want, except that I now have an array that contains the same instance of a counter 5 times, instead of an array of 5 new counters. IE when I run the code

counter_arr = Array.new(5, Counter.new(0)) 
counter_arr[0].tick
counter_arr.each do |c|
    puts(c.starting_value)
end

I output

instead of

I was wondering, what is the "ruby-esque" way to initialize an array with multiple new instances of an object?

Don't forget syntax features like @starting_value += 1. That's usually less verbose and avoids typographical errors. — tadman
– tadman, Commented Aug 2, 2016 at 15:37
This code is pretty much line-for-line identical to the example in the "Common gotchas" section of the docs for Array::new which answer this very question. — Jörg W Mittag
– Jörg W Mittag, Commented Aug 2, 2016 at 15:42

tadman · Accepted Answer · 2016-08-02 15:41:58Z

5

One of the first major stumbling blocks people encounter when learning Ruby if they're unfamiliar with a language that uses object reverences pervasively is how these work.

An array is a collection of references to zero or more other objects. These objects are not necessarily unique, and in some cases they are all identical. You are creating such an object here:

counters = Array.new(5, Counter.new(0))

This creates a singular Counter object and populates all 5 slots of the array with it. This comes about because arguments to methods are evaluated once before the method is called. You can test this:

counters.map(&:object_id)

That returns the unique object ID for each object in the array. They'll be random values, each process is different, but they will be identical.

The way to fix this is to use the block initializer:

counters = Array.new(5) do
  Counter.new(0)
end

That doesn't insert the same object, but the result of evaluating that block each time, and since that initializes a new Counter object, the objects will be unique.

One way to tidy this up is to adjust your Counter object to have a sane default:

class Counter
  def initialize(initial = nil)
    @value = initial.to_i
  end

  def tick
    @value += 0
  end
end

This has the advantage of accepting arbitrary values, even those that aren't necessarily the right type. Now Counter.new('2') will work with that value being converted automatically. This is the fundamental principle of Duck Typing. If it can give you a number, it's as good as a number.

answered Aug 2, 2016 at 15:41

tadman

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6 Comments

dustinroepsch Over a year ago

thanks! That code was literally the first 21 lines of ruby I've ever written, so I appreciate all improvements.

tadman Over a year ago

Hope that works out for you. Seems like you're already off and running, which is good to see.

Cary Swoveland Over a year ago

Very clear answer. Perhaps def initialize(initial=0); @value = initial; end.

tadman Over a year ago

@CarySwoveland I try to default to nil as sometimes values of that sort sneak through inadvertently. = 0 gives a sort of false confidence that it will be forced to 0 by default, but it's only 0 if the argument is not specified, a subtle but important distinction.

Cary Swoveland Over a year ago

You don't want an "inadvertent" nil to raise an exception?

|

Ursus · Accepted Answer · 2016-08-02 15:36:13Z

2

Try

counter_arr = Array.new(5) { Counter.new(0) }

answered Aug 2, 2016 at 15:36

Ursus

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Comments

Aleksei Matiushkin · Accepted Answer · 2016-08-02 15:44:49Z

1

counter_arr = ([-> { Counter.new(0) }] * 5).map &:call

answered Aug 2, 2016 at 15:44

Aleksei Matiushkin

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4 Comments

tadman Over a year ago

That's a pretty crazy solution, but it is at least novel.

dustinroepsch Over a year ago

I think the other answers provide cleaner solutions, but I am still curious as to what is going on here

sawa Over a year ago

It is Ursus's answer expressed inside-out.

Aleksei Matiushkin Over a year ago

Well, first of all I upvoted @tadman’s answer, and then I recalled that I’m the “just-out-of-curiosity-clown” here. So: the Counter constructor is wrapped withing a proc to delay it’s execution, than an array of size 1 is created out of this proc, then this array is being spread to have 5 elements using Array#* and finally it’s mapped to actual counters by invoking procs.

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