Array element is modified

Question

Here is my challenge.

a = ["1", "2"]
b = ["3", "4"]
a << b
a # => ["1","2",["3","4"]]

If I modify the value of b[0], a also is changed.

b[0] = "5"
a # => ["1","2",["5","4"]]

After pushing b into a, b was modified. Why is a changed, and how can I fix it?

Your second question is not clear. What do you mean by fixing it? — sawa
– sawa, Commented Jan 13, 2016 at 7:42
I assume you want %w(1 2 3 4) as the result. If I'm right, you can use a.push(*b), and in this way, a will not change with b. — Aetherus
– Aetherus, Commented Jan 13, 2016 at 7:48
Say you enter a house in a blue T-shirt, then change into a red T-shirt. If anyone takes a photo of the house, would you expect a house with someone in a blue T-shirt, or a house with someone in a red T-shirt? "But it was me that changed clothes, why did the house change?" The house didn't, but its contents did. — Amadan
– Amadan, Commented Jan 13, 2016 at 7:57
This visualisation is instructive (Python instead of Ruby, but otherwise exactly the same). — Amadan
– Amadan, Commented Jan 13, 2016 at 8:03

Wand Maker · Accepted Answer · 2016-01-13 07:18:25Z

4

The array instance that is referenced by b and the one that is pushed into a, both are same instance - hence, modifying one will result in changes seen in other as well.

You can try:

a << b.dup

so that a copy of b is pushed into a.

answered Jan 13, 2016 at 7:18

Wand Maker

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