Convert a string to an existing class instance

Question

Given a class instance and a string, how do I convert the string to refer to the instance?

class Room
  def enter
    puts "Welcome!"
  end
end

# Rooms are predefined
lounge = Room.new
kitchen = Room.new
study = Room.new

puts "Which room would you like to go to?"
print "> "
room = gets.strip

# User types "lounge"

room.enter # => undefined method `enter' for "lounge":String (NoMethodError)

I understand why I'm getting NoMethodError, but I haven't been able to work out how to convert the room string to refer to the existing instance of Room named lounge.

score 1 · Accepted Answer · 2012-10-23 16:13:23Z

1

room_name = gets.strip.capitalize
room = Kernel.const_get(room_name)
room.enter

If you require "active_support/all" (part of Rails) you can do this:

room = gets.strip.camelize.constantize
room.new.enter

Edit:

As the Tin Man pointed out, require "active_support/core_ext/string/inflections" will be more efficient.

edited Oct 23, 2012 at 16:13

answered Oct 23, 2012 at 16:04

user1454117

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4 Comments

the Tin Man Over a year ago

All of ActiveSupport is a big chunk of code that won't necessarily be needed. Why not show how to get just the string manipulation chunks?

user1454117 Over a year ago

Didn't realize you could do that.

Nick Over a year ago

This doesn't seem to work if the class instance is already defined. It works great when creating new instances: e.g. Object.const_get("Room").new.enter, but not when referring to existing ones: Object.const_get("lounge").enter # => wrong constant name lounge (NameError). I think this is because the Room class is a constant, but lounge is not.

the Tin Man Over a year ago

"require "active_support/core_ext/string" will be more efficient". No, require 'active_support/core_ext/string/inflections' will be.

score 1 · Accepted Answer · 2012-10-23 16:16:14Z

1

perhaps trying to map rooms and get them by key?

class Room

  def enter
    puts "Welcome!"
  end
end

# Rooms are predefined
rooms = %w[lounge kitchen study].inject({}) { |f,c| f.update c => Room.new }

puts "Which room would you like to go to?"
print "> "
if room = rooms[gets.strip]
  room.enter
end

Or even simpler:

class Room

  def initialize room_type
    @room_type = room_type
  end

  def enter
    return puts 'Unsupported room type' unless %w[
      lounge kitchen study
    ].include?(@room_type)
    puts "Welcome to #{@room_type}!"
  end

end

puts "Which room would you like to go to?"
print "> "
room = Room.new(gets.strip)
room.enter

edited Oct 23, 2012 at 16:16

answered Oct 23, 2012 at 16:01

user904990

Comments

Victor Moroz · Accepted Answer · 2012-10-23 16:55:28Z

1

class Room
  attr_reader :name

  def initialize(name)
    @name = name
  end

  def enter
    puts "Welcome to #{@name}!"
  end
end

# Rooms are predefined
rooms = ["lounge", "kitchen", "study"].map(&Room.method(:new))

puts "Which room would you like to go to?"
print "> "
name = gets.strip

# Just an example (not using find to avoid nil.enter)
p rooms.select{ |room| room.name == name }.map(&:enter)

answered Oct 23, 2012 at 16:55

Victor Moroz

9,2351 gold badge21 silver badges23 bronze badges

Comments

xing · Accepted Answer · 2012-10-23 16:16:08Z

0

You can use eval method to dynamically execute a piece of ruby script. In this case eval(room) will give you the room instance.

However this is very dangerous method as it allows the end user to execute code. To protect against code execution you'd better verify it's one of the room variable defined:

eval(room).enter if local_variables.include?(room.to_sym)

edited Oct 23, 2012 at 16:16

answered Oct 23, 2012 at 16:01

xing

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Comments

Nick · Accepted Answer · 2012-10-24 06:47:08Z

0

I ended up storing each Room object in a hash as a class variable on initialization. This lets you refer to each instance via the hash:

class Room
  @@rooms = {}

  def initialize(name)
    @@rooms.store name, self
  end

  def rooms
    @@rooms
  end

  def enter
    puts "Welcome!"
  end
end

# Rooms are predefined
lounge = Room.new('lounge')
kitchen = Room.new('kitchen')
study = Room.new('study')

puts "Which room would you like to go to?"
print "> "
new_room = gets.strip

lounge.rooms[new_room].enter if lounge.rooms.has_key? new_room

answered Oct 24, 2012 at 6:47

Nick

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