reduce application

reduce.rb

# the common thinking when you are transforming some data to a non-nil value is use map

a = { foo: 'bar', bar: 'baz' }

[:foo, :bar, :baz, :foobar].map { |k| a[k] }
# => ['bar', 'baz', nil, nil]

# to drop nil cases
[:foo, :bar, :baz, :foobar].map { |k| a[k] }.compact
# => ['bar', 'baz']

[:foo, :bar, :baz, :foobar].reduce([]) do |l, k|
  l << a[k] if a.has_key?(k)
  l
end