ruby,immutability , Initializing an array with identical elements: == operator says arrays are the same, but they behave differently. Why?

Question:

Tag: ruby,immutability

Here's my code:

a=["foo","foo","foo"]
b=["foo"]*3
a==b # => true

a.each{|i| i<<"bar"}
b.each{|i| i<<"bar"}
a==b # => false

I get what I expect for a:

["foobar", "foobar", "foobar"]

but for b, I get this:

["foobarbarbar", "foobarbarbar", "foobarbarbar"]
1. Why is this happening? Is it a bug?
2. Is there a way of filling an array with many identical strings that avoids this problem?

Answer:

It's not a bug, it's just that the lines

a=["foo","foo","foo"]
b=["foo"]*3

Are not the same. The second is inserting the SAME OBJECT three times into the b array. In the a array, you have three different objects. In all cases, the objects are strings with the text "foo".

You can confirm this by examining the object ids.

a[0].object_id == a[1].object_id
=> false

b[0].object_id == b[1].object_id
=> true

So when you mutate the object with << "bar" you are mutating the same object three times (in the case of array b)

To populate an array with separate instances of the same string, do...

Array.new(3) { "foo" }

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