Struct: Ruby’s Quickie Class

Let’s say you have Player and BasketballTeam classes that are defined and used as follows:

	class Player
	attr_accessor :name, :number
	def initialize(name, number)
	@name = name
	@number = number
	end
	end
	class BasketballTeam
	attr_accessor :player1, :player2, :player3, :player4, :player5
	def initialize(player1, player2, player3, player4, player5)
	@player1 = player1
	@player2 = player2
	@player3 = player3
	@player4 = player4
	@player5 = player5
	end
	def starting_lineup
	str = "Ladies and Gentlemen, here is the starting lineup!\n"
	5.times do |num|
	player = self.send("player#{num + 1}")
	str += "\n##{player.number}, #{player.name}!\n"
	end
	str
	end
	end
	team = BasketballTeam.new(Player.new("Magic Johnson", 15), Player.new("Michael Jordan", 9),
	Player.new("Larry Bird", 7),Player.new("Charles Barkley", 14),Player.new("Patrick Ewing", 6))
	puts team.starting_lineup

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basketball_team.rb

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In this case, since there are always exactly 5 players, I don’t want to pull out an array every time and write team.players[0], and instead I’ve chosen to use 5 similarly named instance variables, so I can do team.player1. This looks nice, but also isn’t ideal. If I want to access player n, this starts to get ugly: team.send("player#{n}").

Well, here’s the good news: as usual, Ruby has a better way for you to do it. Introducing: the Struct class! Structs fall somewhere between full-fledged Ruby classes and arrays/hashes, and are excellent for generating classes which are mostly variable storage containers with a particular number of items, with a small number of methods. Here is how we would refactor our code from before:

	Player = Struct.new(:name, :number)
	BasketballTeam = Struct.new(:player1, :player2, :player3, :player4, :player5) do
	def starting_lineup
	"Ladies and Gentlemen, here is the starting lineup!\n" +
	self.collect {|player| "\n##{player.number}, #{player.name}!\n"}.join
	end
	end
	team = BasketballTeam.new(Player.new("Magic Johnson", 15), Player.new("Michael Jordan", 9),
	Player.new("Larry Bird", 7),Player.new("Charles Barkley", 14),Player.new("Patrick Ewing", 6))
	puts team.starting_lineup

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basketball_team_struct.rb

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Huh? Where did all the code go?

Struct.new is a really cool method that takes symbols as arguments and returns – no, it’s not an object, it’s a class!!! (Well, technically all Ruby classes are objects too, but we’re going to deliberately ignore that for now.) It takes each symbol, makes it an instance variable, gives it setter and getter methods, and adds it to the initialize method in the order specified. So it’s doing a lot of work for you, just for adding the symbol there. The optional block at the end (see how BasketballTeam is created with a block but Player isn’t?) specifies any methods you want to add to the struct. If you have a lot of these, Struct probably isn’t for you. But if it’s just one or two simple methods, then Struct may still be a good idea.

An examination of Struct’s instance methods reveals its similarity to Array and Hash. Here are my favorites:

Method	Description and Correlatives
#members	like Hash#keys, returns an array containing the instance variable names
#values	like Hash#values, returns an array containing the instance variable values
#length, #size	like Hash#size or Array#size, the number of instance variables
#each	similar to Hash#each, goes through each instance variable’s value
#[member] (e.g. team["player1"] or team[:player1])	similar to Hash#[], access by instance variable name
#[index] (e.g. team[0])	similar to Array#[], access by variable index in #members

NOTE: You can also write team[0] = Player.new("Magic Johnson", 15)

Of course, you are also able to get team.player1 because it attr_accessor’ed everything for you.

Because Struct defines an #each method and includes Enumerable, you can use any of the Enumerable methods on its properties. So you can cycle, check if team.any? {|player| player.name == "Michael Jordan"}, inject, or find the team.max_by(&:number), among others. You can also modify all contained values pretty easily: team.each{|player| player.number += 1} (in case you needed to bump up everyone’s number for some reason). And if the IOC is insisting you sort your players by jersey number, just team.sort_by(&:number) and you’re all set! Patrick Ewing, with jersey #6, is now team[0], a.k.a. team.player1.

One downside of Struct as opposed to Arrays is that you can’t push/pop/unshift/shift, because the size is fixed from the beginning.

TL;DR A struct is somewhere between a regular object and a hash/array. It’s an awesome data structure when you

• know exactly what it needs to hold
• want to be able to access your data in a variety of useful ways
• need to define just a small number of custom methods (or none at all)
• and just don’t want to write much boilerplate code while doing it!

P.S. Check out this post from Steve Klabnik about how incorporating structs into your regular class definitions can make your debugging much easier due to Struct’s handy #to_s method.

P.P.S. Robert Klemme helpfully notes that, unlike hashes, struct[“something”] will raise an error if there is no @something variable. This can be helpful if you want to detect certain types of input problems.

P.P.P.S. Here’s the output from the code above (using structs or regular classes), if you’re desperately interested:

Ladies and Gentlemen, here is the starting lineup!

#15, Magic Johnson!

#9, Michael Jordan!

#7, Larry Bird!

#14, Charles Barkley!

#6, Patrick Ewing!