Ruby: Dynamically Linked Arrays

Question

I'm having some confusion about populating a new array with an existing array. It seems like if I set one array equal to the other, they become dynamically linked for the rest of the code. Is this a compiler attribute or am I not using the correct functions to store the values of one array in another? I hope this isn't an extremely basic question.

main_arr = [1, 2, 3, 4, 5]
temp_arr = main_arr
puts "Main: " + main_arr.to_s 
puts "Temp: " + temp_arr.to_s

main_arr.clear
puts "Main: " + main_arr.to_s 
puts "Temp: " + temp_arr.to_s

Output:

Main: [1, 2, 3, 4, 5]
Temp: [1, 2, 3, 4, 5]
Main: []
Temp: []

If a is your array, a.dup will create a "shallow" copy. Suppose, for example, a = [[1,2], [3,4]]. Then b = a.dup #=> [[1, 2], [3, 4]]. So far, so good. Now lets add an element to b: b << 'dog' #=> [[1, 2], [3, 4], "dog"]. a is unchanged: a #=> [[1,2], [3,4]]. Consider b[1][0] #=> 3. Now let's change that element of b and see what happens to a: b[1][0] = 'cat'; b #=> [[1, 2], ["cat", 4], "dog"]; a #=> [[1, 2], ["cat", 4]]. Not what you were expecting? That's because dup creates a "shallow" copy of a. — Cary Swoveland
– Cary Swoveland, Commented Jun 17, 2016 at 4:15

Ursus · Accepted Answer · 2016-06-16 21:57:51Z

1

It's true, in your second line you are making temp_array point to the array object pointed by main_arr. If you want to make a copy of that array you can do

b = a.dup

answered Jun 16, 2016 at 21:57

Ursus

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4 Comments

Sean Over a year ago

Ah perfect! It's odd that this isn't covered in the Ruby documentation for arrays...

jmschles Over a year ago

That's because #dup is on Object: ruby-doc.org/core-2.3.1/Object.html#method-i-dup

Cary Swoveland Over a year ago

You can find the owner of an instance method like this: Array.instance_method(:dup).owner #=> Kernel, or [].method(:dup).owner #=> Kernel. (Actually, Array.instance_method(:dup) #=> #<UnboundMethod: Array(Kernel)#dup> is enough, as the owner is shown in parenthesis.) Kernel's instance methods are documented in Object (see second paragraph), which can be a bit confusing. (Kernel's module methods are documented in Kernel.)

Sean Over a year ago

I read through the Object documentation and I know understand what's happening with clone and dup. I appreciate not only the help, but the supporting documentation.

mrvncaragay · Accepted Answer · 2016-06-16 21:58:01Z

0

when you do this: temp_arr = main_arr it means that temp_arr is pointing to main_arr address. use temp_arr = main_arr.dup to copy it instead.

main_arr = [1, 2, 3, 4, 5]
temp_arr = main_arr **means** temp_arr -> [1, 2, 3, 4, 5]

so, when you do this main_arr.clear -> [] and also temp_arr -> []

answered Jun 16, 2016 at 21:58

mrvncaragay

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Comments

Orlando · Accepted Answer · 2016-06-16 21:58:33Z

0

Try doing:

main_arr = [1,2,3,4,5]
temp_arr = main_arr.dup

main_arr.clear
puts main_arr # => []
puts temp_arr # => [1,2,3,4,5]

answered Jun 16, 2016 at 21:58

Orlando

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