Add Map With Index Over An Array as a ruby til

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

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+# 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)