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| -rw-r--r-- | ChangeLog | 5 | ||||
| -rw-r--r-- | doc/syntax/control_expressions.rdoc | 59 |
2 files changed, 64 insertions, 0 deletions
@@ -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 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. + |