Ruby 2.3: Cannot duplicate an array. Original array modified

Question

I'm performing operations on arrays and I'm running into some issues. I duplicated array_1 assigning it to array_2. When I did operations on array_2, the uniq method modifies the original array.

array_3 is what I intended accomplish, but I don't understand why the operations to get there modified array_1

I need an explanation why this behavior occurs, and what I can do to prevent this from happening.

array_1 = [["Ed","2",],["Ann","2"],["Sue","2"],["Ed","3",],["Ann","3"],["Sue","3"]]

array_2 = array_1.dup
array_2 = array_2.uniq(&:first)

array_3=[]
array_2.each do |a2|
  a2.pop
  array_3.push(a2)
end

puts array_3
=> [["Ed"], ["Ann"], ["Sue"]]

puts array_1
=> [["Ed"], ["Ann"], ["Sue"], ["Ed", "3"], ["Ann", "3"], ["Sue", "3"]]

ruby.about.com/od/advancedruby/a/deepcopy.htm

matt
– matt

2016-05-03 04:28:13 +00:00
Commented May 3, 2016 at 4:28 — matt
– matt, Commented May 3, 2016 at 4:28

Community · Accepted Answer · 2020-06-20 09:12:55Z

3

The problem is this line:

array_2 = array_1.dup

From the docs:

dup

Produces a shallow copy of obj—the instance variables of obj are copied, but not the objects they reference.

So array_2 consists of pointers to the same subarrays that are in array_1. Now pop mutates its array. So when you pop a subarray in array_2, it affects the same subarray in array_1.

You'll get the result you seem to expect if you say

array_2 = array_1.map(&:dup)

Personally, though, I would change

a2.pop
array_3.push(a2)

to

array_3.push(a2[0])

I don't see what you gain by mutating the subarrays (though perhaps something is going on that you have not told us).

edited Jun 20, 2020 at 9:12

CommunityBot

11 silver badge

answered May 3, 2016 at 4:21

matt

540k97 gold badges943 silver badges1.3k bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Jordan Running Over a year ago

Please don't advise people to do Marshal.load(Marshal.dump(array_1)), or at least mention that it's almost always the wrong solution. In this case array_1.map(&:dup) would suffice.

ctilley79 Over a year ago

this is part of a larger program. I need to keep the sub arrays intact. I'm using these examples to learn the inner workings of ruby when a problem stumps me

matt Over a year ago

That is why I said that perhaps something is going on that you have not told us.

Collectives™ on Stack Overflow

Ruby 2.3: Cannot duplicate an array. Original array modified

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related