added extension exercise summary

Author: geoknee

_sessions/c3s4/9_hash_array_conversion_solution.md

@@ -0,0 +1,37 @@
+---
+title: (Ext) Exercise summary
+---
+It's easy to convert a `hash` into an `array`.
+{% highlight ruby %}
+h = {'a'=>1, 'b'=>2, 'c'=>3}
+
+a = h.to_a
+# => [["a", 1], ["b", 2], ["c", 3]] 
+{% endhighlight %}
+To convert back from an `array` to a `hash`, use 
+{% highlight ruby %}
+h=Hash[a]
+# => {"a"=>1, "b"=>2, "c"=>3} 
+{% endhighlight %}
+
+You can unpack a `hash` into its keys and values with the `.keys` and `.values` methods
+
+{% highlight ruby %}
+h = {'one'=>1, 'two'=>2, 'three'=>3}
+
+keys = h.keys
+#=> ["one", "two", "three"] 
+values = h.values
+#=> [1, 2, 3] 
+
+
+{% endhighlight %}
+
+and put it back together with 
+
+{% highlight ruby %}
+h = Hash[keys.zip(values)]
+
+# => {"one"=>1, "two"=>2, "three"=>3} 
+{% endhighlight %}
+