I was going to write up another method in the Ruby standard library, but was going to use the include? in some of the examples, so I decided I should just start with include?.

The #include? method is part of the Enumerable module, and when passed and object, it returns true if the object is in the enum.

This works not only for arrays:

[1, 2, 3, 5, 8, 13, 21].include? 4
#=> false
[1, 2, 3, 5, 8, 13, 21].include? 5
#=> true

But also strings:

'banana'.include? 's'
#=> false
'banana'.include? 'a'
#=> true

And maps:

{a: 1, b: 2, c: 3}.include? :a
#=> true
{a: 1, b: 2, c: 3}.include? :q
#=> false
{a: 1, b: 2, c: 3}.include? 1
#=> false

Which if you notice checks if the object is one of the keys in the map.

And because it uses the == operator, it works on coercing datatypes, so we are able to find a floating point version of a number in a list of integers:

[1, 2, 3, 5, 8, 13, 21].include? 1.0
#=> true
[1, 2, 3, 5, 8, 13, 21].include? 1.1
=> false

And the beauty of this is that if you want this behavior on one of your classes or modules, you just include the Enumerable module and implement the #each method, and the Enumerable module will take care of this behavior for you.