Set an array equal to another, then change the first array without affecting the second

Question

I have an array 'a' and I later set a new array 'b' equal to 'a'. When I remove an element from either array, they both change rather than the explicitly referenced array. I think this is because they're pointing to the same underlying object?

a= ["cat","hat","dog"]
b= a
a.delete_at(0)
print a
>> a = ["hat","dog"
print b
>> b = ["hat","dog"]

How can I set two variables equal but allow them to be independent for later operations? I'm looking for behaviour like this:

a= ["cat","hat","dog"]
b= a
a.delete_at(0)
print a
>> a = ["hat","dog"
print b
>> b = ["cat","hat","dog"]

Agis · Accepted Answer · 2014-01-19 11:20:35Z

3

Use Object#dup to create a duplicate of the original array:

a = ["cat","hat","dog"]
b = a.dup # b now points to a duplicate of a, not a itself

b # => ["cat","hat","dog"]
b.delete_at(0)

b # => ["hat","dog"]
a # => ["cat","hat","dog"]

To verify this behavior:

a = [1, 2, 3]
b = a
a.object_id == b.object_id # => true, a and b point to the same objects

a = [1, 2, 3]
b = a.dup
a.object_id == b.object_id # => false, a and b point to different objects

edited Jan 19, 2014 at 11:20

answered Jan 19, 2014 at 11:06

Agis

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1 Comment

proximate_cause Over a year ago

I just noticed that the "DUP" method doesn't work when doing string manipulations. For example, if I do b[0].upcase! the first element of both arrays is changed. Any insight into that?

Cary Swoveland · Accepted Answer · 2014-01-19 18:44:26Z

It is perhaps worth noting that if

a = [["cat", "pig"], "hat", "dog"] # => [["cat", "pig"], "hat", "dog"]
b = a.dup                          # => [["cat", "pig"], "hat", "dog"]

then

a[0].delete_at(0)
a                                  # => [["pig"], "hat", "dog"] 
b                                  # => [["pig"], "hat", "dog"]

Here you'd need to do this:

a = [["cat", "pig"], "hat", "dog"]
b = a.dup 
b[0] = a[0].dup
a[0].delete_at(0)
a                                  # => [["pig"], "hat", "dog"] 
b                                  # => [["cat", "pig"], "hat", "dog"]

Now suppose a were much more complicated, say, an array of hashes whose values are arrays of hashes. Obviously, one must take care in making a copy of a that will be unaffected by the deletion of some deeply-nested element of a. What you would want is called a "deep copy", as opposed to the "shallow" copy made by Object#dup and Object#clone.

Fortunately, there is an easy way to make a deep-copy of any object, using Marshal#dump and Marshal#load:

b = Marshal.load(Marshal.dump(a))
a[0].delete_at(0)
a                                  #  => [["pig"], "hat", "dog"]
b                                  #  => [["cat", "pig"], "hat", "dog"]

As an aside, Marshal is also used to save an object to file for later retrieval:

File.open('myfile', 'w+') do |f|  
  Marshal.dump(a, f)  
end  

File.open('myfile') do |f|  
  c = Marshal.load(f)  
end  
c                                  #  => [["cat", "pig"], "hat", "dog"]

A word of caution: Marshal files are not not portable among all versions of Ruby.

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