How to check if a value exists in an array in Ruby

You're looking for include?:

>> ['Cat', 'Dog', 'Bird'].include? 'Dog'
=> true

There is an in? method in ActiveSupport (part of Rails) since v3.1, as pointed out by @campaterson. So within Rails, or if you require 'active_support', you can write:

'Unicorn'.in?(['Cat', 'Dog', 'Bird']) # => false

OTOH, there is no in operator or #in? method in Ruby itself, even though it has been proposed before, in particular by Yusuke Endoh a top notch member of ruby-core.

As pointed out by others, the reverse method include? exists, for all Enumerables including Array, Hash, Set, Range:

['Cat', 'Dog', 'Bird'].include?('Unicorn') # => false

---

Note that if you have many values in your array, they will all be checked one after the other (i.e. O(n)), while that lookup for a hash will be constant time (i.e O(1)). So if you array is constant, for example, it is a good idea to use a Set instead. E.g:

require 'set'
ALLOWED_METHODS = Set[:to_s, :to_i, :upcase, :downcase
                       # etc
                     ]

def foo(what)
  raise "Not allowed" unless ALLOWED_METHODS.include?(what.to_sym)
  bar.send(what)
end

A quick test reveals that calling include? on a 10 element Set is about 3.5x faster than calling it on the equivalent Array (if the element is not found).

A final closing note: be wary when using include? on a Range, there are subtleties, so refer to the doc and compare with cover?...

Try

['Cat', 'Dog', 'Bird'].include?('Dog')