Add Map With Index Over An Array as a ruby til

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/).
 
-_850 TILs and counting..._
+_851 TILs and counting..._
 
 ---
 
@@ -669,6 +669,7 @@ _850 TILs and counting..._
 - [Last Raised Exception In The Call Stack](ruby/last-raised-exception-in-the-call-stack.md)
 - [Limit Split](ruby/limit-split.md)
 - [Listing Local Variables](ruby/listing-local-variables.md)
+- [Map With Index Over An Array](ruby/map-with-index-over-an-array.md)
 - [Mock Method Chain Calls With RSpec](ruby/mock-method-chain-calls-with-rspec.md)
 - [Mocking Requests With Partial URIs Using Regex](ruby/mocking-requests-with-partial-uris-using-regex.md)
 - [Navigate Back In The Browser With Capybara](ruby/navigate-back-in-the-browser-with-capybara.md)

ruby/map-with-index-over-an-array.md

@@ -0,0 +1,29 @@
+# Map With Index Over An Array
+
+The [`#map`](https://devdocs.io/ruby~2.5/enumerable#method-i-map) method on its
+own allows you to interact with each item of an array, producing a new array.
+
+```ruby
+[1,2,3].map { |item| item * item }
+#=> [1,4,9]
+```
+
+If you also want access to the index of the item, you'll need some help from
+other enumerable methods. As of Ruby 1.9.3, you can chain on
+[`#with_index`](https://devdocs.io/ruby~2.5/enumerator#method-i-with_index):
+
+```ruby
+[1,2,3].map.with_index { |item, index| item * index }
+#=> [0,2,6]
+```
+
+This method has the added benefit of allowing you to specify the starting value
+of the index. It normally starts with `0`, but you could just as easily start
+at `1`:
+
+```ruby
+[1,2,3].map.with_index(1) { |item, index| item * index }
+#=> [1,4,9]
+```
+
+[source](https://stackoverflow.com/a/11280903/535590)