The Perl Flip Flop Operator

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The Perl Flip Flop Operator

November 29, 2010 by Jared

Mike Taylor dismisses Perl with a pithy reference to a section of its excellent documentation¹. For some reason, I mis-remembered that he was complaining about the flip-flop operator rather than context in general.

So, I’ve come to defend the flip-flop operator, and the opposition hasn’t turned up! Oh well, never mind.

In scalar context, “..” returns a boolean value. The operator is bistable, like a flip-flop, and emulates the line-range (comma) operator of sed, awk, and various editors. Each “..” operator maintains its own boolean state. It is false as long as its left operand is false. Once the left operand is true, the range operator stays true until the right operand is true, AFTER which the range operator becomes false again.

Scanning Logfiles

So, say your logfile looks something like this:

... 100,000 lines ...
10:22:25.279 The first interesting line
... 30 more interesting lines ...
10:22:25.772 Another interesting line
10:22:25.772 The last interesting line
10:22:25.779 And then this line isn't interesting any more
... 100,000 lines ...

If you specify the beginning timestamp and end timestamp then you will get one uninteresting line which you can strip with head -n-1.

And that is it. Pretty easy eh?

jared@localhost $ cat flip-flop.muse \
> | perl -ne 'print if /^10:22:25.279/ .. /^10:22:25.779/' \
> | head -n-1 \
> | mail jared
10:22:25.279 The first interesting line
... 30 more interesting lines ...
10:22:25.772 Another interesting line
10:22:25.772 The last interesting line

Notes:

In a regex I often use an unescaped period (.) to match a period if it doesn’t matter like here

And for anyone thinking useless use of cat… it’s deliberate.

1. With impressive inconsistency, he later on says that Perl is a contender to be His Favourite Language which is why the alternative title for this post was Why Mike Taylor is not my Favourite Blogger.

Just kidding Mike.

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Posted in Perl | Tagged flip-flop, logfiles, perl operator | 8 Comments

8 Responses

on November 29, 2010 at 07:13 Tweets that mention The Perl Flip Flop Operator « A Curious Programmer -- Topsy.com

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on November 29, 2010 at 12:16 LeoNerd

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

$ <flip-flop.muse \
perl -ne ….

Shell redirections can come before the command.
on November 29, 2010 at 13:53 Jared

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 ‘…’
on November 29, 2010 at 16:53 Evan Carroll

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
on November 29, 2010 at 17:13 Rhomboid

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.
on November 29, 2010 at 21:11 Jared

@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!
on November 29, 2010 at 22:53 Tom Legrady

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
on November 30, 2010 at 06:34 Jared

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

Thanks.