Ruby seem to reference a variable instead of coping it's value

Question

Why does the value in my data_dummy hash increase? I’d like to use it to initiate another hash with zero values!

fau[f.label][:hash] = data_dummy #  ==>{"name 1" => 0, "name 2" => 0} but in the second loop it contains data from the first loop e.g. {"name 1" => 2, "name 2" => 0}

When using the string instead of variable dummy_data the code works as expected.

fau[f.label][:hash] = {"name 1" => 0, "name 2" => 0}

I can't do that because 'name X' is changing....

That's strange to me!

complete code

fau = {}
series = []
labels = [{:value => 0, :text => ''}]

data_dummy = {}
source.each do |c|
  data_dummy[c.name] = 0
end
i = 0
data_dummy.each do |k,v|
  i += 1
  labels.push({:value => i, :text => k})
end

source.each do |s|
  logger.debug "~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~"
  logger.debug "Source: '#{s.name}'|'#{fault_labels[s.fault_id.to_s].label}' => #{s.cnt}"
  con_name = s.name #TODO: Cut name (remove location like left,right, ...)
  f = fault_labels[s.fault_id.to_s]
  unless fau.has_key?(f.label)
    # init faults-hash 'fau'
    fau[f.label] = {:total => 0, :hash => {}, :color => f.color, :name => f.label} #, :data => []
    # add all connector_names as keys with value = 0
    logger.debug "init :hash with #{data_dummy}" #  ==>{"name 1" => 0, "name 2" => 0} but in the second loop it contains data from the first loop e.g. {"name 1" => 2, "name 2" => 0}
    fau[f.label][:hash] = data_dummy
    # this way the number of incidents are all in the same order for each fault (first dimension key)
    # and all get at least a value of 0
 end
  logger.debug "Count up fau['#{f.label}'][:total] = #{fau[f.label][:total]} + #{s.cnt} (where connector '#{s.name}' and fault '#{f.label}')"
  logger.debug "Count up fau['#{f.label}'][:hash]['#{con_name}'] = #{fau[f.label][:hash][con_name]} + #{s.cnt}"
  fau[f.label][:total] += s.cnt
  fau[f.label][:hash][con_name] += s.cnt
  logger.debug "result :hash with #{fau[f.label][:hash].inspect}}"
end

score 5 · Accepted Answer · edited May 23 '17 at 12:15

5

Because Ruby hashes, like all Ruby objects, are references and copying one, such as hash2 = hash1 only creates a copy of the reference. Modifying hash2 will modify hash1, as really, they are just different aliases for the same thing.

You want to use the clone method instead.

hash2 = hash1.clone

See also How do I copy a hash in Ruby?

Note that even this only creates a shallow copy, if you have a nested hash (such as myhash = {"key1" => "value1", "key2" => {"key2a" => "value2a"}}), you will have to make a deep copy. According to Wayne Conrad's answer to the question above, the way to do that is this:

def deep_copy(o)
  Marshal.load(Marshal.dump(o))
end

edited May 23 '17 at 12:15

Community

1
1

answered Sep 27 '12 at 23:26

Dondi Michael Stroma

4,668
18
21

2

They're all references, you just don't notice with things like numbers and symbols because they're immutable. – mu is too short Sep 27 '12 at 23:35
I didn't know that Ruby references - shame on me ;-) THANKS! With .clone all works fine now! – BBQ Chef Sep 28 '12 at 09:42

score 2 · Answer 2 · answered Sep 27 '12 at 23:31

If you want to make a copy of the hash, you need to use the dup method:

foo = {"name 1" => 0, "name 2" => 0}
bar = foo
foo["name 2"] += 1
foo
=> {"name 2"=>1, "name 1"=>0}
bar
=> {"name 2"=>1, "name 1"=>0}

baz = foo.dup
foo["name 2"] += 1
foo
=> {"name 2"=>2, "name 1"=>0}
baz
=> {"name 2"=>1, "name 1"=>0}

Ruby seem to reference a variable instead of coping it's value

2 Answers2