Comments on: The Perl Flip Flop Operator

By: Jared

Jared — Tue, 30 Nov 2010 06:34:15 +0000

Good point in this specific case Tom. I’ve amended the post to make the utility more obvious.

Thanks.

By: Tom Legrady

Tom Legrady — Mon, 29 Nov 2010 22:53:49 +0000

It’s the unneccessary use of the flip-flop operator that concerns me.

jared@localhost $ cat flip-flop.muse \
> | perl -ne ‘print if /^10:22:25/ \
> | mail jared

By: Jared

Jared — Mon, 29 Nov 2010 21:11:24 +0000

@Evan – I don’t find the behaviour surprising – rather than having global state, I’d say it’s acting like a closure, or an object. If it didn’t behave like this when called multiple times I’d be more surprised.

@Rhombold – that is a neat trick. That’s two things I’ve learned today. Cheers!

By: Rhomboid

Rhomboid — Mon, 29 Nov 2010 17:13:25 +0000

You can get rid of the head -n-1 stuff by checking the return value of the flip-flop. The return value is a number representing the count, and when the ending condition is met (i.e. the flip-flop is transitioning from true to false) then this number is returned with a trailing E0 such that it’s still numeric but can be distinguished, that is “5E0″ is still parsed as the number 5 but it’s different than “5″, so:

$ seq 1 100 | perl -ne ‘print if (/^22/ .. /^26/) =~ /^\d+$/’
22
23
24
25

The last line is omitted.

By: Evan Carroll

Evan Carroll — Mon, 29 Nov 2010 16:53:22 +0000

See this question on StackOverflow for some added esoterism:
http://stackoverflow.com/questions/2143554/is-perls-flip-flop-operator-bugged-it-has-global-state-how-can-i-reset-it

By: Jared

Jared — Mon, 29 Nov 2010 13:53:55 +0000

Hi LeoNerd,

That’s a neat trick that I wasn’t aware of, thanks!

But that wasn’t the reason, no. I always process files starting with cat because I usually need to process multiple files and build up my pipelines iteratively. E.g.

cat log2010* | grep blah | awk ‘{ … }’ …
cat log2010* | awk ‘/blah/ { … }’ | perl -ne …
cat log2010* | perl -ne ‘…’

By: LeoNerd

LeoNerd — Mon, 29 Nov 2010 12:16:20 +0000

Your deliberate use of cat; would that perhaps be to keep the filename first in a left-to-right reading? If so, you know you can do

$ perl -ne ….

Shell redirections can come before the command.

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Mon, 29 Nov 2010 07:13:14 +0000

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