Creating a Hash with values as arrays and default value as empty array [duplicate]

Question

I want to create a Hash in Ruby with default values as an empty array

So, I coded

x = Hash.new([])

However, when I try to push a value into it

x[0].push(99)

All the keys get 99 pushed into that array. How do I resolve this?

Answer 1

Lakshmi is right. When you created the Hash using Hash.new([]), you created one array object.

Hence, the same array is returned for every missing key in the Hash.

That is why, if the shared array is edited, the change is reflected across all the missing keys.

Using:

Hash.new { |h, k| h[k] = [] }

Creates and assigns a new array for each missing key in the Hash, so that it is a unique object.

Answer 2

h = Hash.new{|h,k| h[k] = [] }

h[0].push(99)

This will result in {0=>[99]}

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When Hash.new([]) is used, a single object is used as the default value (i.e. value to be returned when a hash key h[0] does not return anything), in this case one array.

So when we say h[0].push(99), it pushes 99 into that array but does not assign h[0] anything. So if you output h you will still see an empty hash {}, while the default object will be [99].

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Whereas, when a block is provided i.e. Hash.new{|h,k| h[k] = [] } a new object is created and is assigned to h[k] every time a default value is required.

h[0].push(99) will assign h[0] = [] and push value into this new array.