From 038c535e18549740c34d63efc3326c8e12379317 Mon Sep 17 00:00:00 2001
From: drbrain <drbrain@b2dd03c8-39d4-4d8f-98ff-823fe69b080e>
Date: Wed, 16 Jan 2013 22:50:54 +0000
Subject: 	* doc/syntax/control_expressions.rdoc (Flip-Flop):  Added a
 section on 	  the flip-flop.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@38852 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
---
 doc/syntax/control_expressions.rdoc | 59 +++++++++++++++++++++++++++++++++++++
 1 file changed, 59 insertions(+)

(limited to 'doc/syntax')

diff --git a/doc/syntax/control_expressions.rdoc b/doc/syntax/control_expressions.rdoc
index 18dd40b676..e3fc19d72d 100644
--- a/doc/syntax/control_expressions.rdoc
+++ b/doc/syntax/control_expressions.rdoc
@@ -418,3 +418,62 @@ longer true, now you will receive a SyntaxError when you use +retry+ outside
 of a +rescue+ block.  See {Exceptions}[rdoc-ref:syntax/exceptions.rdoc]
 for proper usage of +retry+.
 
+== Flip-Flop
+
+The flip-flop is rarely seen conditional expression.  It's primary use is
+for processing text from ruby one-line programs used with <code>ruby -n</code>
+or <code>ruby -p</code>.
+
+The form of the flip-flop is an expression that indicates when the
+flip-flop turns on, <code>..</code> (or <code>...</code>), then an expression
+that indicates when the flip-flop will turn off.  While the flip-flop is on it
+will continue to evaluate to +true+, and +false+ when off.
+
+Here is an example:
+
+
+  selected = []
+
+  0.upto 10 do |value|
+    selected << value if value==2..value==8
+  end
+
+  p selected # prints [2, 3, 4, 5, 6, 7, 8]
+
+In the above example the on condition is <code>n==2</code>.  The flip-flop
+is initially off (false) for 0 and 1, but becomes on (true) for 2 and remains
+on through 8.  After 8 it turns off and remains off for 9 and 10.
+
+The flip-flop must be used inside a conditional such as +if+, +while+,
++unless+, +until+ etc. including the modifier forms.
+
+When you use an inclusive range (<code>..</code>) the off condition is
+evaluated when the on condition changes:
+
+  selected = []
+
+  0.upto 5 do |value|
+    selected << value if value==2..value==2
+  end
+
+  p selected # prints [2]
+
+Here both sides of the flip-flop are evaluated so the flip-flop turns on and
+off only when +value+ equals 2.  Since the flip-flop turned on in the
+iteration it returns true.
+
+When you use an exclusive range (<code>...</code>) the off condition is
+evaluated on the following iteration:
+
+  selected = []
+
+  0.upto 5 do |value|
+    selected << value if value==2...value==2
+  end
+
+  p selected # prints [2, 3, 4, 5]
+
+Here the flip-flop turns on when +value+ equals 2 but doesn't turn off on the
+same iteration.  The off condition isn't evaluated until the following
+iteration and +value+ will never be two again.
+
-- 
cgit v1.2.3