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* doc/syntax/control_expressions.rdoc (Flip-Flop): Added a section on

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the flip-flop.


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drbrain 2013-01-16 22:50:54 +00:00
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@ -1,3 +1,8 @@
Thu Jan 17 07:50:26 2013 Eric Hodel <drbrain@segment7.net>
* doc/syntax/control_expressions.rdoc (Flip-Flop): Added a section on
the flip-flop.
Thu Jan 17 06:59:51 2013 Eric Hodel <drbrain@segment7.net>
* doc/syntax/control_expressions.rdoc (if Expressions): Fixed markup

59
doc/syntax/control_expressions.rdoc
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@ -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.