Add Updating Values In A Map as an elixir til

README.md

@@ -7,7 +7,7 @@ variety of languages and technologies. These are things that don't really
 warrant a full blog post. These are mostly things I learn by pairing with
 smart people at [Hashrocket](http://hashrocket.com/).
 
-_458 TILs and counting..._
+_459 TILs and counting..._
 
 ---
 
@@ -94,6 +94,7 @@ _458 TILs and counting..._
 - [Replace Duplicates In A Keyword List](elixir/replace-duplicates-in-a-keyword-list.md)
 - [Reversing A List](elixir/reversing-a-list.md)
 - [String Interpolation With Just About Anything](elixir/string-interpolation-with-just-about-anything.md)
+- [Updating Values In A Map](elixir/updating-values-in-a-map.md)
 - [Word Lists For Atoms](elixir/word-lists-for-atoms.md)
 
 ### Git

elixir/updating-values-in-a-map.md

@@ -0,0 +1,41 @@
+# Updating Values In A Map
+
+When working with maps in any language, you often need a way to update
+key-value pairs. Furthermore, you will need a way to handle keys that are
+not already present in the map, generally associating some default value.
+
+In Elixir, the `Map` module provides the `get_and_update/3` function as a
+way of accomplishing such a task.
+
+You need to provide the map, the key to be fetched and update, and a
+function for _transforming_ (or updating) the existing value.
+
+Let's use a score counting example to see it in action:
+
+```elixir
+> scores = %{}
+%{}
+# jake scores a point
+> {_, scores} = Map.get_and_update(scores, :jake, fn(x) -> {x, (x || 0) + 1} end)
+{nil, %{jake: 1}}
+# chris scores a point
+> {_, scores} = Map.get_and_update(scores, :chris, fn(x) -> {x, (x || 0) + 1} end)
+{nil, %{chris: 1, jake: 1}}
+# jake scores another point
+> {_, scores} = Map.get_and_update(scores, :jake, fn(x) -> {x, (x || 0) + 1} end)
+{1, %{chris: 1, jake: 2}}
+# final scores
+> scores
+%{chris: 1, jake: 2}
+```
+
+We use `(x || 0) + 1` as the updated value. This is a simple way of
+providing a default value for when the key is not already present in the
+map.
+
+The update function is expected to return a tuple with the _original_ value
+and the _updated_ value.
+
+See [the
+docs](http://elixir-lang.org/docs/stable/elixir/Map.html#get_and_update/3)
+for more details.