Add Pass A Block To Count as a ruby til.

README.md

@@ -205,6 +205,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 - [Next And Previous Floats](ruby/next-and-previous-floats.md)
 - [Override The Initial Sequence Value](ruby/override-the-initial-sequence-value.md)
 - [Parallel Bundle Install](ruby/parallel-bundle-install.md)
+- [Pass A Block To Count](ruby/pass-a-block-to-count.md)
 - [Passing Arbitrary Methods As Blocks](ruby/passing-arbitrary-methods-as-blocks.md)
 - [Passing Arguments To A Rake Task](ruby/passing-arguments-to-a-rake-task.md)
 - [Percent Notation](ruby/percent-notation.md)

ruby/pass-a-block-to-count.md

@@ -0,0 +1,23 @@
+# Pass A Block To Count
+
+Ruby's [`Enumerable`](http://ruby-doc.org/core-2.2.3/Enumerable.html) module
+comes with the method `#count` for determining how many items are in an
+array or hash.
+
+```ruby
+> [1,2,3].count
+=> 3
+> {a: 1, b: 2}.count
+=> 2
+```
+
+The `#count` method has a trick up its sleeve though. It can take a block
+with a predicate that returns `true` or `false`. It essentially acts like
+`#select` returning the count rather than the array subset itself.
+
+```ruby
+> [1,2,3].count { |x| x.odd? }
+=> 2
+> {a: 1, b: 2}.count { |(x,y)| y < 0 }
+=> 0
+```