When would a Ruby flip-flop be useful?

Question

I think I understand how a flip-flop works thanks to this tutorial, but the example there is contrived just for teaching. Can anyone give an example of how you have actually used or would use a flip-flop?

Edit: I'm looking for a real-world application, not just another demonstration. What problems can this tool solve?

Answer 1

Here's an example (taken from a rubycentral.com article) where you print out only certain lines from a file:

file = File.open("ordinal")
while file.gets
    print if ($_ =~ /third/) .. ($_ =~ /fifth/)
end

This assumes that you have a file with the following contents:

first
second
third
fourth
fifth
sixth

The program will only print out:

third
fourth
fifth

The idea is that it the value is true until the left-hand event happens, and then stays true until the right-hand event happens. If used properly this can be a nice piece of syntactic sugar, but you need to be careful to make things readable.

Edit: fixed code that rampion found objectionable.

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