Is there a smart way of checking whether a custom object is within an array?

Question

Given I've got an array arr of instances of a custom class My_class. This class has got a couple of different instance variables. Let's call them :name (a String), :is_processed (a Boolean) and :date (a DateTime). All of them are readable.

What is the best way of checking, whether another given instance of My_class (call it newbe) is already in the array based on the content of the aforementioned instance variables?

Using arr.include?(newbe) will not work as this compares the object IDs, doesn't it?
Can I override the way Array#include? compares two objects?

Speaking in code

class My_class
  attr_reader :name, :is_processed, :date

  def initialize(_name, _proc, _d)
    @name = _name
    @is_processed = _proc
    @date = _d
  end
end

somewhere else

first = My_class.new("First", false, DateTime.new(2001,2,3,4,5,6))
second = My_class.new("Second", true, DateTime.new(2001,2,3,4,5,6))
third = My_class.new("Third", false, DateTime.new(2001,2,3,3,2,1))
newbe = My_class.new("Second", true, DateTime.new(2001,2,3,4,5,6))

arr = [first, second, third]
arr.include?(newbe)  # => false but should be true

steenslag · Accepted Answer · 2012-01-20 14:51:04Z

3

You want to compare instances of you class, so include comparable, define a <=> method and you have added these methods to your class instances:

<, <=, ==, >, >=, between? (#include? uses #== ).

require 'date'
class My_class
  include Comparable
  attr_reader :name, :is_processed, :date

  def initialize(_name, _proc, _d)
    @name = _name
    @is_processed = _proc
    @date = _d
  end

  def <=>(other)
    [self.name, self.is_processed, self.date]<=>[other.name, other.is_processed, other.date]
  end
end
first = My_class.new("First", false, DateTime.new(2001,2,3,4,5,6))
second = My_class.new("Second", true, DateTime.new(2001,2,3,4,5,6))
third = My_class.new("Third", false, DateTime.new(2001,2,3,3,2,1))
newbe = My_class.new("Second", true, DateTime.new(2001,2,3,4,5,6))

arr = [first, second, third]
p arr.include?(newbe)  # => true
#you could do arr.sort, but they are sorted allready by accident...

edited Jan 20, 2012 at 14:51

answered Jan 19, 2012 at 21:51

steenslag

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1 Comment

Torbjörn Over a year ago

Thank you for this answer. I'm going to implement this approach as I think it's the most generic one.

d11wtq · Accepted Answer · 2012-01-19 21:39:08Z

2

Use any? with a block.

my_arr.any? { |o| o.kind_of?(MyClass) }

You can also use detect or find if you want the matched object back. These are all standard methods on Enumerable, which most collection-ish classes mix in.

answered Jan 19, 2012 at 21:39

d11wtq

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2 Comments

Torbjörn Over a year ago

Thanks for this quick answer. As far as I don't want to know the index of the matching item (or a count of how many items matches) this is probably the nicest way. However, I should substitute o.kind_of(MyClass) with (o.name == newbe.name && o.is_processed == newbe.is_processed && o.date == newbe.date). PS: I'll wait for some more answers before accepting one.

Andrew Grimm Over a year ago

! my_arr.grep(MyClass).empty? should also work, based on classes implementing ===. might be a slight headscratcher for the maintainer though.

Alex D · Accepted Answer · 2012-01-19 21:46:18Z

2

You should consider whether it would make sense to redefine == for your custom class, like this:

class My_class
  def ==(other)
    (other.is_a? My_class) && 
    (other.name == @name)  &&
    (other.is_processed == @is_processed) &&
    (other.date == @date)
  end
end

If you do this, Array#include? will work the way you want.

answered Jan 19, 2012 at 21:46

Alex D

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