Add Up And Down With Integers as a ruby til

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

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+# 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.