# FAQ: Data Structures - Using Hash.new

This community-built FAQ covers the “Using Hash.new” exercise from the lesson “Data Structures”.

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## FAQs on the exercise Using Hash.new

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Hi!
I’m having trouble figuring out how to use . Hash.new

I don’t know why this syntax doesn’t work.

my_hash = Hash.new.capitalize! “pets” => “Mimi”

puts my_hash[“pets”]

The error message explains it…

undefined method `capitalize!’ for {}:Hash

which means that we cannot use that syntax.

``````my_hash = Hash.new
my_hash["pets".capitalize!] = "Mimi"
puts my_hash    # {"Pets"=>"Mimi"}
``````
1 Like

After trying all of the variables suggested here and those I thought of. The answer is a very simple:

pets = Hash.new

2 Likes

OMG, thank you for this. This just helped and once seen it makes sense

what is the purpose of using Hash.new?
I understand using hashes that asign values to keys. But what is Hash.new achieving if it isn’t assigning value to anything?

The `.new()` method on the Hash object can take a default value to assign to every new Hash.

``````x = Hash.new(0)
x['word'] += 1
puts x['word']    #  1
``````

We see this employed in a word frequency hash, for instance.

2 Likes

Is there a benefit to using obj = Hash.new rather than obj = {} when they do the same thing? It seems obj = {} would be more efficient considering the difference in the number of characters.

The number of characters is not a sign of efficiency; to determine that we examine time complexity, especially of loops, and space complexity in terms of how much memory is consumed.

Declaring a literal hash is okay when the values are random data. The advantage we gain with Hash.new is an initial default value for each additional member.

Eg.

``````hist = Hash.new(0)
``````

This will come up in the Build a Histogram exercise.