http://qs321.pair.com?node_id=765480

wu-lee has asked for the wisdom of the Perl Monks concerning the following question:

I want to use a flip-flop to truncate a list after a certain value. This should be easy, right? After all, getting a sub-sequence works: 
  # Get everything between B and D:
  perl -lwe 'print join " ", grep scalar(/B/../D/), qw(A B C D E F)'
  # prints:
  # B C D

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This doesn't work, because of the (documented in perlop) magic comparison with $. when the conditions are constants: 
  # Get everything between up to D:
  perl -lwe 'print join " ", grep scalar(1../D/), qw(A B C D E F)'
  # prints:
  # Use of uninitialized value in range (or flip) at -e line 1.
  # Use of uninitialized value in range (or flip) at -e line 1.
  # Use of uninitialized value in range (or flip) at -e line 1.
  # Use of uninitialized value in range (or flip) at -e line 1.
  # Use of uninitialized value in range (or flip) at -e line 1.
  # Use of uninitialized value in range (or flip) at -e line 1.

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Ok, fair enough. But this doesn't work correctly, and there aren't any warnings: 
  # Get everything up to and D:
  perl -lwe 'print join " ", grep scalar(/./../D/), qw(A B C D E F)'
  # prints:
  # A B C D E F

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In fact, as flip-flops return the sequence numbers of their matches, we can double check what these are: 
  # Get everything up to D:
  perl -lwe 'print join " ", map scalar(/./../D/), qw(A B C D E F)'
  # prints:
  # 1 2 3 4E0 1 2

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Eh? Why does it start back at 1 after 4E0?

Can anyone shed any light on this? (I'm using Perl 5.8.8 on Ubuntu.)

Replies are listed 'Best First'.
Re: Flip-flop won't DWIM
by ikegami (Patriarch) on May 21, 2009 at 16:52 UTC

    This doesn't work, because of the (documented in perlop) magic comparison with $. when the conditions are constants:

    Fix: 

    perl -lwe'$true=1; print for grep $true../D/, qw(A B C D E F)'

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    Note that it will flip for "E" (like your last version). What you probably intended is: 

    perl -lwe'my @a=qw(A B C D E F); print for grep ($_==1) .. ($a[$_]=~/D
+/), 0..$#a'

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    Or without a flip-flop: 

    perl -lwe'for (qw( A B C D E F )) { print; last if /D/ }'

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[reply]
[d/l]
[select]
      Thanks - in the end I also decided a flip-flop wasn't really necessary and used a more pedestrian loop. This post was mainly to see if there was in fact a nice way of using a flip flop in this case; I think ??../ D/ fits the bill.
[reply]
        Not really 
        >perl -le"for (1..2) { print for grep scalar(??../D/), ('A'..'F'); }"
A
B
C
D

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[reply]
[d/l]
Re: Flip-flop won't DWIM
by akho (Hermit) on May 21, 2009 at 15:49 UTC
    The /./ matches E, so flip-flop starts over.
[reply]
      So use the match-once operator (no need for an actual pattern; the empty pattern works fine): 
      print join ' ', grep scalar(??../D/), ('A'..'F');

      [download]
      Caution: Contents may have been coded under pressure.
[reply]
[d/l]
        Aha! Finally, a use for the match-once operator! And it seems to be tailor made for this problem. Bravo!
[reply]
[reply]
        Yes. However, List::MoreUtils is not in core, so it's not something I want to rely on in the current circumstance.
[reply]
      Right. Of course, silly me.

      Ok, so this does what I want: 

      perl -lwe '$x = 1; print join " ", grep { scalar($x++../D/) } qw(A B C
+ D E F)'
# prints:
# A B C D

      [download]
      Although this is not particularly neat. Is there a better way?
[reply]
[d/l]
        Your code prints "A B C D E F" for me, and I cannot see why it would print anything else.

        This seems to work if you insist on using flip-flop: 

        $x = 1; print join " ", grep { $x..(($x=0) or /D/) } qw(A B C D E F);

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        However, you could copy stuff from List::MoreUtils.
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[d/l]
        Actually, that doesn't, but this does: 
         perl -lwe '$x = 0; print join " ", grep { scalar(!$x++../D/) } qw(A B
+ C D E F)'
 # prints:
 # A B C D

        [download]
[reply]
[d/l]
        The "++" does nothing.
[reply]

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