Question

I have a 2D array containing many instances of a class. The class contains 4 arrays. I would like to save and load the 2D array to/from disk using Marshal. I have successfully used Marshal for this purpose with other 2D arrays containing classes, but those classes did not contain an array. Here is the definition of the class giving me trouble.

class Light
       attr_accessor :R,:G,:B,:A

       def initialize(i)

            @R = Array.new(4, i)
            @G = Array.new(4, i)
            @B = Array.new(4, i)
            @A = Array.new(4, i)

       end

       @R
       @G
       @B
       @A

end

I have tried defining my own marshal functions in the Light class:

def marshal_dump
    {'R' => @R,'G' => @G,'B' => @B,'A' => @A}
end


def marshal_load(data)
    self.R = data['R']
    self.G = data['G']
    self.B = data['B']
    self.A = data['A']
end

Here is the creation of the 2D array containing this class

def createLightMap(width,height)
     a = Array.new(width) { Light.new(0.7) }
     a.map! { Array.new(height) { Light.new(0.7) } }
     return a
end

@lightMap = createLightMap(10,10)

Here is how I save and load

#save
File.open('lightData','w') do |file|
     Marshal.dump(@lightMap, file)
end

#load
@lightMap = if File.exists?('lightData')
                  File.open('lightData','w') do |file|
                       Marshal.load(file)
                  end
             else
                  puts 'no light data found'
             end

Upon load, I receive the error “in ‘load’: dump format error (unlinked, index: -96) (Argument Error)”

I have tried with and without custom dump/load marshal functions. I am using jruby 1.5.1, ruby 1.8.7

Answer 1

I don’t think it’s the Marshal dump/load that is the problem, it’s probably just the file I/O. This works fine for me (without custom marshaling):

class Light
  # You might want to downcase these variables as capitalized 
  # variables in Ruby generally denote constants
  attr_accessor :R,:G,:B,:A

  def initialize(i)
    @R = Array.new(4, i)
    @G = Array.new(4, i)
    @B = Array.new(4, i)
    @A = Array.new(4, i)
  end

  def ==(other)
    @R == other.R && @G == other.G && @B == other.B && @A == other.A
  end
end

# Method names are generally underscored / snake cased
# (JRuby is even smart enough to map this to Java's camel casing).
# Also should probably attach this method to a class or module to prevent
# polluting global namespace
def create_light_map(width,height)
  a = Array.new(width) { Light.new(0.7) }
  # Note you don't need explicit returns - the last value evaluated is the return value
  a.map { Array.new(height) { Light.new(0.7) } } # You can also lose the ! on map
end

# Same casing style applies to variables
light_map = create_light_map(10,10)
# => [[#<Light:0x5ec736e4 @A=[0.7, 0.7, 0.7, 0.7], ...

# Note with marshaled data you should probably open file in binary mode
File.open('/tmp/lightData','wb') { |f| f.write(Marshal.dump(light_map)) }
# => 5240

light_map_demarshaled = File.open('/tmp/lightData','rb') { |f| Marshal.load(f.read) }
# => [[#<Light:0x6a2d0483 @A=[0.7, 0.7, 0.7, 0.7], ...

light_map_demarshaled == light_map
# => true