Hash#slice like method that returns nil if given key are not present

I would use a reduce of the terms you are interested in:

hash = {a: 1, b: 2, d: 4}
[:a, :c, :d].reduce({}) {|h, k| h[k] = hash[k]; h }

Here's my solution; it creates a new Hash from the keys array with nil values, and merges that with the results of the slice:

> Hash[keys.zip].merge(hash.slice(*keys))
 => {:a=>1, :c=>nil, :d=>4}