Add Assoc For Hashes as a ruby til.

README.md

@@ -108,6 +108,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 ### ruby
 
 - [Are They All True?](ruby/are-they-all-true.md)
+- [Assoc For Hashes](ruby/assoc-for-hashes.md)
 - [Comparing Class Hierarchy Relationships](ruby/comparing-class-hierarchy-relationships.md)
 - [Create an Array of Stringed Numbers](ruby/create-an-array-of-stringed-numbers.md)
 - [Destructuring Arrays In Blocks](ruby/destructuring-arrays-in-blocks.md)

ruby/assoc-for-hashes.md

@@ -0,0 +1,19 @@
+# Assoc For Hashes
+
+Ruby's Hash class comes with a method, `assoc`, which is good for grabbing
+both the key and value from a hash. If the given key matches it returns a
+two element array with the key and value.
+
+```ruby
+> stuff = {a: 1, b: 2, c: 3}
+=> {a: 1, b: 2, c: 3}
+> stuff.assoc(:c)
+=> [:c, 3]
+```
+
+If a key is used that doesn't exist in the hash, then it simply returns nil.
+
+```ruby
+> {}.assoc(:c)
+=> nil
+```