I'd like to merge these two arrays:
a = { :a => 'a', :b => 'b', :d => 'd' }
b = { :a => '1', :b => 'b', :c => 'c' }
Into a hash that looks like this:
{:a => ["a", "1"], :b => ["b", "b"], :c => [nil, "c"], :d => ["d", nil] }
This obviously doesn't work:
p a.merge(b) { |k, v1, v2| [v1, v2] } # {:c=>"c", :a=>["a", "1"], :b=>["b", "b"], :d=>"d"}
It's because Hash#merge call the block only for the duplicate keys.
a = { :a => 'a', :b => 'b', :d => 'd' }
b = { :a => '1', :b => 'b', :c => 'c' }
Hash[(a.keys|b.keys).map { |key|
[key, [a.fetch(key, nil), b.fetch(key, nil)]]
}]
# => {:a=>["a", "1"], :b=>["b", "b"], :d=>["d", nil], :c=>[nil, "c"]}
# In Ruby 2.1+
(a.keys|b.keys).map { |key|
[key, [a.fetch(key, nil), b.fetch(key, nil)]]
}.to_h
(a.keys | b.keys).each_with_object({}) { |k,h| h[k] = [a[k], b[k]] }
#=> {:a=>["a", "1"], :b=>["b", "b"], :d=>["d", nil], :c=>[nil, "c"]}
This is easy to generalize for an arbitrary number of hashes.
arr = [{ :a => 'a', :b => 'b', :d => 'd' },
{ :a => '1', :b => 'b', :c => 'c' },
{ :b => '0', :c => 'b', :d => 'c' }]
arr.reduce([]) { |ar,h| ar | h.keys }
.each_with_object({}) { |k,h| h[k] = arr.map { |g| g[k] } }
#=> {:a=>["a", "1", nil], :b=>["b", "b", "0"],
# :d=>["d", nil, "c"], :c=>[nil, "c", "b"]}
#each_with_object is a nice method. I like it.. +1