Add Pass A Block To Count as a ruby til.

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

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+# 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
+```