Ruby gives different results for different approaches to array of arrays

Question

I am trying to append some values to a particular array in an array of arrays, like this:

a1 = [[]] * 2
a1[0] << -1
a1 # => [[-1], [-1]]
a2 = [[], []]
a2[0] << -1
a2 # => [[-1], []]
[[]] * 2 == [[], []] # => true

a2 has the expected value while a1 seems to be wrong. What I was expecting is a1 to have the value [[-1], []] since I changed a1[0] & not a1[1].

In the first case, the two elements are actually the same array. When you change one, the other sees the change. — Sergio Tulentsev
– Sergio Tulentsev, Commented Dec 17, 2015 at 13:00
@sawa I have edited the answer to express what I expected. However, now I understand the issue as shivam has pointed out. — kamalbanga
– kamalbanga, Commented Dec 17, 2015 at 13:12
@kamalbanga in that case I was expecting my answer to be accepted :( — shivam
– shivam, Commented Dec 18, 2015 at 6:00
@shivam: When I tried to accept your answer, there was yet time to be able to do that. Later I forgot. I have accepted it now :). — kamalbanga
– kamalbanga, Commented Dec 21, 2015 at 11:29

shivam · Accepted Answer · 2015-12-17 13:01:21Z

11

You should use:

a1 = Array.new(2) { [] }

[[]]*2 is for repetition and is just repeating the same object [] twice.

To support of my above point:

a1 = [[]] * 2
a1.map(&:object_id)
#=> [26686760, 26686760]   # same object ids

a3 = Array.new(2) { [] }
a3.map(&:object_id)
#=> [23154760, 23154680]   # different object ids

answered Dec 17, 2015 at 13:01

shivam

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Marek Lipka · Accepted Answer · 2015-12-17 13:01:14Z

2

Both subarrays in your a1 are the same Array object.

answered Dec 17, 2015 at 13:01

Marek Lipka

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