Ruby array of hashes unexpected behaviour

Question

I was trying to create an array of hashes, to which i have already found some excellent solutions here:

However while trying it out on my own, i found some behavior which i didn't understand.

Creating an array of hashes in IRB:

array_hashes = Array.new(7, Hash.new)

Now, on trying to assign key, value pairs to the array:

array_hashes[1]["hello"] = 200

I got the following output in console:

=>[{"hello"=>200}, {"hello"=>200}, {"hello"=>200}, {"hello"=>200}, {"hello"=>200}, {"hello"=>200}, {"hello"=>200}]

The same key , value was repeated across all the array elements, similar results when i tried to assign another key, value to a single array element

array_hashes[3]["world"] = 300
=>[{"hello"=>200, "world"=>300}, {"hello"=>200, "world"=>300}, {"hello"=>200, "world"=>300}, {"hello"=>200, "world"=>300}, {"hello"=>200, "world"=>300}, {"hello"=>200, "world"=>300}, {"hello"=>200, "world"=>300}]

Can anyone explain the reason for this, specifically why are hash values repeated across all the array elements even on being assigned to a single element. Thanks!

Ruby Version used: 1.9.3, tried on Windows 7 and OS X Yosemite

This has been asked many times before here on SO. The short answer is: read the docs for Array::new, they even contain the exact example you are asking about. — Jörg W Mittag
– Jörg W Mittag, Commented Dec 10, 2014 at 17:55

Maurício Linhares · Accepted Answer · 2014-12-10 17:41:49Z

3

It's repeated because what this code does:

array_hashes = Array.new(7, Hash.new)

Is this:

hash = Hash.new
array_hashes = [hash, hash, hash, hash, hash, hash, hash]

So it's the same object included 7 times in the array.

But what you wanted to do was:

array_hashes = Array.new(7) { Hash.new }

answered Dec 10, 2014 at 17:41

Maurício Linhares

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