Does Ruby have a `Pair` data type?

Question

Sometimes I need to deal with key / value data.

I dislike using Arrays, because they are not constrained in size (it's too easy to accidentally add more than 2 items, plus you end up needing to validate size later on). Furthermore, indexes of 0 and 1 become magic numbers and do a poor job of conveying meaning ("When I say 0, I really mean head...").

Hashes are also not appropriate, as it is possible to accidentally add an extra entry.

I wrote the following class to solve the problem:

class Pair
  attr_accessor :head, :tail

  def initialize(h, t)
      @head, @tail = h, t
  end
end

It works great and solves the problem, but I am curious to know: does the Ruby standard library comes with such a class already?

Looks like a linked list to me. Is that the meaning you're trying to convey? — fylooi, Feb 07 '17 at 15:52
@flyooi - No, although you could probably do that with my implementation if you wanted to. — Rick, Feb 07 '17 at 16:06
`Pair = Struct.new(:head, :tail)` is a shorter way to write this class. — steenslag, Feb 07 '17 at 21:25

score 4 · Answer 1 · edited May 23 '17 at 11:45

4

No, Ruby doesn't have a standard Pair class.

You could take a look at "Using Tuples in Ruby?".

The solutions involve either using a similar class as yours, the Tuples gem or OpenStruct.

Python has tuple, but even Java doesn't have one: "A Java collection of value pairs? (tuples?)".

edited May 23 '17 at 11:45

Community

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answered Feb 07 '17 at 15:41

Eric Duminil

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score 3 · Accepted Answer · answered Feb 07 '17 at 17:04

3

You can also use OpenStruct datatype. Probably not exactly what you wanted, but here is an implementation ...

require 'ostruct'

foo = OpenStruct.new
foo.head = "cabeza"
foo.tail = "cola"

Finally,

puts foo.head
 => "cabeza"

puts foo.tail
 => "cola"

answered Feb 07 '17 at 17:04

Som Poddar

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score 2 · Answer 3 · answered Feb 07 '17 at 16:05

No, there is no such class in the Ruby core library or standard libraries. It would be nice to have core library support (as well as literal syntax) for tuples, though.

I once experimented with a class very similar to yours, in order to replace the array that gets yielded by Hash#each with a pair. I found that monkey-patching Hash#each to return a pair instead of an array actually breaks surprisingly little code, provided that the pair class responds appropriately to to_a and to_ary:

class Pair
  attr_reader :first, :second

  def to_ary; [first, second] end
  alias_method :to_a, :to_ary

  private

  attr_writer :first, :second

  def initialize(first, second)
    self.first, self.second = first, second
  end

  class << self; alias_method :[], :new end
end

score 0 · Answer 4 · answered Nov 02 '18 at 17:22

You don't need a special type, you can use a 2-element array with a little helper to give the pairs a consistent order. E.g.:

def pair(a, b)
  (a.hash < b.hash) ? [a, b] : [b, a]
end

distances = {
  pair("Los Angeles", "New York") => 2_789.6,
  pair("Los Angeles", "Sydney") => 7_497,
}

distances[ pair("Los Angeles", "New York") ]    # => 2789.6
distances[ pair("New York", "Los Angeles") ]    # => 2789.6
distances[ pair("Sydney", "Los Angeles") ]      # => 7497

Does Ruby have a `Pair` data type?

4 Answers4