Add Up And Down With Integers as a ruby til

README.md

@@ -333,6 +333,7 @@ _422 TILs and counting..._
 - [Summing Collections](ruby/summing-collections.md)
 - [Uncaught Exceptions In Pry](ruby/uncaught-exceptions-in-pry.md)
 - [`undef_method` And The Inheritance Hierarchy](ruby/undef-method-and-the-inheritance-hierarchy.md)
+- [Up And Down With Integers](ruby/up-and-down-with-integers.md)
 - [Who Are My Ancestors?](ruby/who-are-my-ancestors.md)
 - [Zero Padding](ruby/zero-padding.md)
 

ruby/up-and-down-with-integers.md

@@ -0,0 +1,31 @@
+# Up And Down With Integers
+
+Ruby's [`Integer`](http://ruby-doc.org/core-2.2.0/Integer.html) class comes
+with an `#upto` and a `#downto` method. Both of these methods can be used to
+iterate from one number up or down to, respectively, another number.
+
+Let's count to 3
+
+```ruby
+> 1.upto(3) { |x| puts x }
+1
+2
+3
+```
+
+This of course can easily and perhaps more idiomatically be accomplished
+with a range and the `#each` method (e.g. `(1..3).each { |x| puts x }`.
+
+We cannot, however, simulate the `#downto` method with a range (at least,
+not very cleanly). So, if you need to count down to something, this is going
+to be the cleanest and clearest way.
+
+```ruby
+> 5.downto(2) { |x| puts x }
+5
+4
+3
+2
+```
+
+The return value for both methods is always the integer we started with.