Copy hash values from a hash into another only where keys exist in both hashes?

user_5783745 0
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How can the values from one hash be copied into another, but only for the keys that exist in both hashes?

Minimal example

h1 = {a: 10, b: 20, c: 30}
h2 = {a: nil, c: nil}

## desired result
{a: 10, c: 30}

I've solved with a 'manual' approach - i.e. {a: h1[:a], c: h1[:c]} - but it's verbose and looks worse the more keys there are. I'm sure there's something (much) more elegant?

ruby
2 Answers

You can do that as follows.

h1 = { a: 10, b: 20, c: 30 }
h2 = { a: nil, c: nil, d: nil }

h1.slice(*h2.keys)
  #=> {:a=>10, :c=>30}

See Hash#slice.

Note that the example above differs from the one in the question in that I've added a key to h2 (:d) that is not present in h1.

user_2569700

The "manual" approach with a loop would look something like this

h1.each do |k, v|
  h2[k] = v if h2.include? k
end

If your goal is to create a new hash, rather than modifying an existing one, you can use filter (called select in older versions of Ruby)

h1.filter { |k, v| h2.include? k }

Or if you want to modify h1 to only have the keys from k2, you can use filter! (called select! in older versions), which works like filter but modifies in-place.

h1.filter! { |k, v| h2.include? k }
user_22886590
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