1

So I am trying to create dynamic variables, and then use them again later without having to type out the full name. The test code I made is as follows 

players = [1, 2, 3, 4]

players.each |player|
  instance_variable_set("@player_#{player}_hand", "foo")
  instance_variable_set("@player_#{player}_value", "bar")
}

puts @player_1_hand # => foo
puts @player_3_value # => bar

players.each { |player| 
puts "@player_#{player}_hand" 
}

# => @player_1_hand
# => @player_2_hand
# => @player_3_hand
# => @player_4_hand

I want to call and/or update these variables dynamically but I don't know how to do that after I initialize them. The documentation I have found has been more confusing than helpful. Can someone explain how to use these correctly?

AnkitG
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Martin
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4 Answers4

8

I recommend that you follow OOP concepts creating a Player object instead of many instance variables, one for each attribute. So, instead of

@player_1_hand
@player_3_value

Try with a Player class

class Player
  attr_accessor :number, :hand, :value
  def initialize(number)
    @number = number
  end
end

And then

p1 = Player.new(1)
p2 = Player.new(2)
p3 = Player.new(3)
p4 = Player.new(4)
[p1,p2,p3,p4].each do |player|
  player.hand = ...
  player.value = ...
end


p1.hand #=> ...
p1.value #=> ...
cmramseyer
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  • This is the correct answer. Thanks for wrangling this into proper Ruby form. One thing you can do is make the initializer take hash-style options (*kwargs*), a hash, or employ the `yield self if block_given?` approach to allow quickly populating all those fields. – tadman Oct 30 '17 at 20:47
  • You could also have an array of Players instead of discrete variables for each one, and iterate the array with `each`. – Tom Oct 30 '17 at 21:12
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    All of those are right approaches, but creating a simple Player class is a good start – cmramseyer Oct 30 '17 at 21:20
2

I want to warn you that the answer I'm going to give you will work for what you are trying to accomplish, but the way you're going about it is akin to using a wrench when you need a hammer. Sure, the wrench might work, but I'd fire any plumber that didn't use the right tools.

players.each { |p| puts instance_variable_get("@player_#{p}_hand") }

More about instance_variable_get can be found at apidock.

In general though, you should stay away from instance variables unless you are creating instances of a class. But for a script like this, stick with normal variables and stay away from instance variables.

anothermh
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2
players = [1, 2, 3, 4]

players.each { |player|
instance_variable_set("@player_#{player}_hand", "foo")
instance_variable_set("@player_#{player}_value", "bar")
}

To call your variables use eval 

players.each { |player| 
puts instance_variable_get("@player_#{player}_hand") 
puts instance_variable_get("@player_#{player}_value")
}

To update your variables use instance_variable_set again

players.each { |player| 
instance_variable_set("@player_#{player}_hand", instance_variable_get("@player_#{player}_hand") + (player + 5).to_s)
puts instance_variable_get("@player_#{player}_hand")
}

all that prints:

foo
bar
foo
bar
foo
bar
foo
bar
foo6
foo7
foo8
foo9
developer_hatch
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  • How'd you overlook `instance_variable_get`? There's absolutely no reason for using `eval` here. It's massive overkill. – tadman Oct 30 '17 at 20:47
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    @tadman it's not about correctness or rubyness here... OP asked a question, specific one, I provided an answer, one that provides a solution. I really don't get your point, obviusly I could uses classes, and all the object stuff super cool, but there is none of those in the OP code, because the code wasn't about object ortiented programming... – developer_hatch Oct 30 '17 at 20:50
  • I mean that `eval` is completely out of place here because [`instance_variable_get`](https://ruby-doc.org/core-2.4.2/Object.html#method-i-instance_variable_get) exists as a counterpart to `instance_variable_set`. Why not pair these up? That's what they're meant for. If you're using `eval` you better have a really good reason, and there's absolutely no reason here. – tadman Oct 30 '17 at 20:52
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    @tadman aaaah I see now, you are right, I modified the answer, you were correct after all, thanks for the patient – developer_hatch Oct 30 '17 at 20:55
1

When a you want an attr_accessor for every property, a Struct instead of a class will make them for you automatically.

Player = Struct.new(:number, :hand, :value)

players = Array.new

(1..4).each do |i|
  player = Player.new(i)
  player.hand = 'something'
  player.value = 'blah'
  players.push player
end

players.each do |player|
  puts "Player #{player.number} has hand #{player.hand} with value #{player.value}"
end

Output:

Player 1 has hand something with value blah
Player 2 has hand something with value blah
Player 3 has hand something with value blah
Player 4 has hand something with value blah
Tom
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