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
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.
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
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
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)'

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'

Or without a flip-flop:

```perl -lwe'for (qw( A B C D E F )) { print; last if /D/ }'
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.
Not really
```>perl -le"for (1..2) { print for grep scalar(??../D/), ('A'..'F'); }"
A
B
C
D
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.
So use the match-once operator (no need for an actual pattern; the empty pattern works fine):
```print join ' ', grep scalar(??../D/), ('A'..'F');

Caution: Contents may have been coded under pressure.
Aha! Finally, a use for the match-once operator! And it seems to be tailor made for this problem. Bravo!
Yes. However, List::MoreUtils is not in core, so it's not something I want to rely on in the current circumstance.
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
Although this is not particularly neat. Is there a better way?
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);
``` perl -lwe '\$x = 0; print join " ", grep { scalar(!\$x++../D/) } qw(A B