ruby unintended variable assignment/change

Question

i'm quite a layman in programming and a noob to ruby but find it useful for my work anyway. Cuirrently I work on a rather large script which brought the following unintended effect:

def my_reduce_method(value_hash,some_keys)
  value_hash.delete(some_keys) 
end

puts all_values
=> all_values

some_values = all_values # start my block with all values (class: hash)

some_values = my_reduce_method(some_values,keys_to_reduce) 
# here only some_values should be effected!

puts all_values
=> some_values

Right in the block there is no damage, but the original all_values is lost! How can I ensure that in a certain code block a certain variable is definitely not changed?

Thank you in advance for any input!!!

Nothing in the code you posted will modify all_values, so I'm guessing something in my_reduce_method is modifying it. — Jordan Running
– Jordan Running, Commented Nov 14, 2013 at 17:50
It's a shallow copy of all_values. That's the problem. my_reduce_method is modifying some_values, which is pointing to the same data that all_values points to. — Plasmarob
– Plasmarob, Commented Nov 14, 2013 at 17:53
There was no good reason for the downvote. Most appropriate question from first-time poster I've seen. — Plasmarob
– Plasmarob, Commented Nov 14, 2013 at 17:58
@Plasmarob, and, as usual, no explanation/justification provided for the downvote. — Cary Swoveland
– Cary Swoveland, Commented Nov 14, 2013 at 18:20
An easy way to make of a deep copy of any object o that can be dup'ed: Marshal.load(Marshal.dump(o)). — Cary Swoveland
– Cary Swoveland, Commented Nov 14, 2013 at 18:28

jaeheung · Accepted Answer · 2013-11-14 19:26:01Z

1

All object assignments in Ruby are reference assignments.

That means, when you do:

some_values = all_values

You're copying the reference(or address) of the object which all_values is referencing(or pointing).

The solution for your case is simple:

some_values = all_values.clone

.dup also works usually (can be different depending on the object).

Another thing to be careful about is, when all_values[:x] has a string and you do:

some_values = all_values.clone
some_values[:x] += 'abc'

This will not change all_values[:x] because some_values[:x] gets (is assigned) a new string object.

But if you do:

some_values = all_values.clone
some_values[:x] << 'abc'

Both all_values[:x] and some_values[:x] change, because they both reference the same string object.

This is the effect of the shallow copy @Plasmarob mentioned.

edited Nov 14, 2013 at 19:26

answered Nov 14, 2013 at 18:11

jaeheung

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11 Comments

Sergio Tulentsev Over a year ago

Strings are references too.

Plasmarob Over a year ago

How kind of you to credit. Someday I'd like to see SO implement a clean interface for people to merge their answers and ownership.

Plasmarob Over a year ago

Ditto, ditto, and ditto.

jaeheung Over a year ago

@Plasmarob My point is, .clone and .dup also make shallow copies. Thanks for the kind comment though.

Holger Just Over a year ago

@jaeheung: Strings are mutable and as such aren't copied when you assign them to a variable. They just get a new reference. Both variables then point to the same String object (as verified by its object_id)

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bertramscharpf · Accepted Answer · 2013-11-14 19:40:32Z

0

Object#clone preserves the frozen attribute and singleton methods. If you don't need those, what happens to your example, Object#dup is sufficient.

answered Nov 14, 2013 at 19:40

bertramscharpf

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