Add Iterate With An Offset Index as a ruby til

Author: jbranchaud

README.md

@@ -10,7 +10,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 For a steady stream of TILs from a variety of rocketeers, checkout
 [til.hashrocket.com](https://til.hashrocket.com/).
 
-_831 TILs and counting..._
+_832 TILs and counting..._
 
 ---
 
@@ -648,6 +648,7 @@ _831 TILs and counting..._
 - [Get Info About Your RubyGems Environment](ruby/get-info-about-your-ruby-gems-environment.md)
 - [Identify Outdated Gems](ruby/identify-outdated-gems.md)
 - [If You Detect None](ruby/if-you-detect-none.md)
+- [Iterate With An Offset Index](ruby/iterate-with-an-offset-index.md)
 - [Ins And Outs Of Pry](ruby/ins-and-outs-of-pry.md)
 - [Invoking Rake Tasks Multiple Times](ruby/invoking-rake-tasks-multiple-times.md)
 - [Last Raised Exception In The Call Stack](ruby/last-raised-exception-in-the-call-stack.md)

ruby/iterate-with-an-offset-index.md

@@ -0,0 +1,35 @@
+# Iterate With An Offset Index
+
+You can iterate over a collection of items with the
+[`#each`](https://devdocs.io/ruby~2.5/enumerator#method-i-each) method. If you
+want to know the index of each item as you go, you can use the
+[`#each_with_index`](https://devdocs.io/ruby~2.5/enumerable#method-i-each_with_index)
+variant.
+
+```ruby
+> ["one", "two", "three"].each_with_index do |item, index|
+    puts "#{item} - #{index}"
+  end
+one - 0
+two - 1
+three - 2
+=> ["one", "two", "three"]
+```
+
+The initial index will always be `0` when using `#each_with_index`.
+
+What about if you want the index value to be offset by some number?
+
+You can use the
+[`#with_index`](https://devdocs.io/ruby~2.5/enumerator#method-i-with_index)
+method on an _enumerator_. It optionally takes an `offset` argument.
+
+```ruby
+> ["one", "two", "three"].each.with_index(1) do |item, index|
+    puts "#{item} - #{index}"
+  end
+one - 1
+two - 2
+three - 3
+=> ["one", "two", "three"]
+```