check if value exists in array in Ruby

Try

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

There is no in operator or #in? method, but 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?...

Use Enumerable#include:

a.include? 'Dog'

If you want to check by a block, you could try any? or all?.

%w{ant bear cat}.any? {|word| word.length >= 3}   #=> true  
%w{ant bear cat}.any? {|word| word.length >= 4}   #=> true  
[ nil, true, 99 ].any?                            #=> true  

Details are here: http://ruby-doc.org/core-1.9.3/Enumerable.html
My inspiration come from here: http://stackoverflow.com/a/10342734/576497

There's the other way around, too!

Suppose the array is [ :edit, :update, :create, :show ] - well perhaps the entire seven deadly/restful sins :)

And further toy with the idea of pulling a valid action from some string - say

my brother would like me to update his profile

Solution

[ :edit, :update, :create, :show ].select{|v| v if "my brother would like me to update his profile".downcase =~ /[,|.| |]#{v.to_s}[,|.| |]/}

Several answers suggest Array#include?, but there is one important caveat: Looking at the source, even Array#include? does perform looping:

rb_ary_includes(VALUE ary, VALUE item)
{
    long i;

    for (i=0; i<RARRAY_LEN(ary); i++) {
        if (rb_equal(RARRAY_AREF(ary, i), item)) {
            return Qtrue;
        }
    }
    return Qfalse;
}

The way to test the word presence without looping is by constructing a trie for your array. There are many trie implementations out there (google "ruby trie"). I will use rambling-trie in this example:

a = %w/cat dog bird/

require 'rambling-trie' # if necessary, gem install rambling-trie
trie = Rambling::Trie.create { |trie| a.each do |e| trie << e end }

And now we are ready to test the presence of various words in your array without looping over it, in O(log n) time, with same syntactic simplicity as Array#include?, using sublinear Trie#include?:

trie.include? 'bird' #=> true
trie.include? 'duck' #=> false