Assign different variable names (which are nil array) through a loop

Question

I have a loop which should assign different variable names depending on filesname which are contained in an array.

Each variable is set as an empty array.

filenames = ['file1', 'file2']

filenames.each do |filename|
  "@data_" + "#{filename}" = []        # used @ as I need these variables externaly from the loop
end

This gives me the following error

syntax error, unexpected '=', expecting keyword_end

What I don't understand is if I use

filenames.each do |filename|
  p "@data_" + "#{filename}"
end

it renders correctly

@data_file1
@data_file2

And if I set an empty array through the loop

filenames.each do |filename|
  @data = []
end
p @data
#=> []

it works too...

Thanks for your help!

+1 for trying to work it out yourself.

Andrew Grimm
– Andrew Grimm

2011-08-15 22:16:52 +00:00
Commented Aug 15, 2011 at 22:16 — Andrew Grimm
– Andrew Grimm, Commented Aug 15, 2011 at 22:16

DigitalRoss · Accepted Answer · 2011-08-15 16:12:03Z

4

I would recommend using a data structure rather than synthesizing instance variables¹ with meta-programming.

Why not just:

@data = filenames.inject({}) { |h, v| h[v] = []; h }
# => { "file1" => [], "file2" => [] }

Now you can just reference @data['file1'] where you want the 'file1' Array object.

^{1. But for the record, here is one way:}

class A
  def f x, y
    self.class.send :attr_accessor, x
    send "#{x}=", y
  end
end

a = A.new
a.f 'new_instance_variable', 12345
p a.new_instance_variable

edited Aug 15, 2011 at 16:12

answered Aug 15, 2011 at 15:58

DigitalRoss

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2 Comments

tokland Over a year ago

Hash[pairs] is perhaps a somewhat cleaner way of creating a hash. Let's hope Ruby gets Enumerable#mash some day and we all agree how to create a hash from an enumerable.

Andrew Grimm Over a year ago

each_with_object is simpler than inject in this case.

Chowlett · Accepted Answer · 2011-08-15 15:59:00Z

2

Simplest solution, if you're sure about the sanitsed-ness of your filenames array, would be:

filenames = ['file1', 'file2']

filenames.each do |filename|
  eval "@data_#{filename} = []"
end

answered Aug 15, 2011 at 15:59

Chowlett

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1 Comment

emboss Over a year ago

You should probably add that this will not work anymore in Ruby 1.9.

maro · Accepted Answer · 2011-08-15 16:07:36Z

2

What you're doing in "@data_" + "#{filename}" = [] is assigning empty Array instance ([]) to a String instance ("@data_file1"), hence syntax error, unexpected '=', expecting keyword_end

You should want to do is to assign [] to instance variable: @data_file, not string: "@data_file".

You can achieve that with instance_variable_set method of class Object:

filenames = ['file1', 'file2']

filenames.each do |filename|
    instance_variable_set("@data_#{filename}".to_sym, [])
end

answered Aug 15, 2011 at 16:07

maro

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Comments

austinbv · Accepted Answer · 2011-08-15 16:01:30Z

1

Why are you using string concatenation and interpolation in the same string...

something like

p "@data_#{filename}"

is the same as your

p "@data_" + "#{filename}"

Now this won't work because you are trying to say string = something else you need to evaluate the string

eval("@data_#{filename} = []")

answered Aug 15, 2011 at 16:01

austinbv

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