Flip-flop won't DWIM

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
[download]

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.
[download]

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
[download]

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
[download]

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.)

Comment on Flip-flop won't DWIM Select or Download Code

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 $true../D/, qw(A B C D E F)'` [download] Note that it will flip for "E" (like your last version). What you probably intended is: `perl -lwe'my @a=qw(A B C D E F); print for grep ($_==1) .. ($a[$_]=~/D +/), 0..$#a'` [download] Or without a flip-flop: `perl -lwe'for (qw( A B C D E F )) { print; last if /D/ }'` [download]	[reply] [d/l] [select]
Re^2: Flip-flop won't DWIM by wu-lee (Beadle) on May 22, 2009 at 11:05 UTC
Thanks - in the end I also decided a flip-flop wasn't really necessary and used a more pedestrian loop. This post was mainly to see if there was in fact a nice way of using a flip flop in this case; I think ??../ D/ fits the bill.	[reply]
Re^3: Flip-flop won't DWIM by ikegami (Patriarch) on May 22, 2009 at 15:11 UTC
Not really `>perl -le"for (1..2) { print for grep scalar(??../D/), ('A'..'F'); }" A B C D` [download]	[reply] [d/l]
Re^4: Flip-flop won't DWIM by Roy Johnson (Monsignor) on May 26, 2009 at 16:22 UTC
Re: Flip-flop won't DWIM by akho (Hermit) on May 21, 2009 at 15:49 UTC
The /./ matches E, so flip-flop starts over.	[reply]
Re^2: Flip-flop won't DWIM by Roy Johnson (Monsignor) on May 21, 2009 at 20:30 UTC
So use the match-once operator (no need for an actual pattern; the empty pattern works fine): `print join ' ', grep scalar(??../D/), ('A'..'F');` [download] Caution: Contents may have been coded under pressure.	[reply] [d/l]
Re^3: Flip-flop won't DWIM by wu-lee (Beadle) on May 22, 2009 at 10:54 UTC
Aha! Finally, a use for the match-once operator! And it seems to be tailor made for this problem. Bravo!	[reply]
Re^2: Flip-flop won't DWIM by akho (Hermit) on May 21, 2009 at 15:57 UTC
Also: List::MoreUtils is your friend.	[reply]
Re^3: Flip-flop won't DWIM by wu-lee (Beadle) on May 21, 2009 at 16:03 UTC
Yes. However, List::MoreUtils is not in core, so it's not something I want to rely on in the current circumstance.	[reply]
Re^2: Flip-flop won't DWIM by wu-lee (Beadle) on May 21, 2009 at 15:58 UTC
Right. Of course, silly me. Ok, so this does what I want: `perl -lwe '$x = 1; print join " ", grep { scalar($x++../D/) } qw(A B C + D E F)' # prints: # A B C D` [download] Although this is not particularly neat. Is there a better way?	[reply] [d/l]
Re^3: Flip-flop won't DWIM by akho (Hermit) on May 21, 2009 at 16:15 UTC
Your code prints "A B C D E F" for me, and I cannot see why it would print anything else. This seems to work if you insist on using flip-flop: `$x = 1; print join " ", grep { $x..(($x=0) or /D/) } qw(A B C D E F);` [download] However, you could copy stuff from List::MoreUtils.	[reply] [d/l]
Re^3: Flip-flop won't DWIM by wu-lee (Beadle) on May 21, 2009 at 16:13 UTC
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]
Re^3: Flip-flop won't DWIM by ikegami (Patriarch) on May 21, 2009 at 16:57 UTC
The "++" does nothing.	[reply]


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