Recursively counting a matching pattern on a single line.

Given a bookmark:

<DT><A HREF="http://www.perl.com/CPAN-local/doc/FAQs/FAQ/PerlFAQ.html" ADD_D
ATE="897592292" LAST_VISIT="982769648" LAST_MODIFIED="982769648" ID="rdf:#$r
sy5Z">PERL FAQ</A>


Wondering if it's possible to have 2 counters that would keep track of the
number of < and > encountered.

Im looking to see if there's a balance of < and >, and scream as soon as
there isnt a balance.

So given the above bookmark, I know that there's 3 <  and  3 >
and the pattern is basically <><><> and not <<>> or >><, etc.

Thanks
Birl

S.A. Birl wrote:
> Given a bookmark:
>
> <DT><A
> HREF="http://www.perl.com/CPAN-local/doc/FAQs/FAQ/PerlFAQ.html"
> ADD_DATE="897592292" LAST_VISIT="982769648" LAST_MODIFIED="982769648"
> ID="rdf:#$rsy5Z">PERL FAQ</A>
>
>
> Wondering if it's possible to have 2 counters that would keep track
> of the
> number of < and > encountered.
>
> Im looking to see if there's a balance of < and >, and scream as soon
> as
> there isnt a balance.
>
> So given the above bookmark, I know that there's 3 <  and  3 >
> and the pattern is basically <><><> and not <<>> or >><, etc.

If you're specifically looking whether the brackets follow the pattern
<><><>, could you just do something like:

my $brackets = join '', $bookmark =~ /[<>]/g;
die "Bad pattern" unless $brackets eq '<><><>';

Assign each line to the $_ variable and try this to get the number of
instances of < and >

The match operator works on $_ by default. If you match on < or > globally
and assign it to an array like so:

@rights = m/\</g;
@lefts = m/\>/g;

You are left with arrays that contain all the matches. Now all you need to
do is count the number of elements in each array like so:

$number_of_rights = $#rights;
$number_of_lefts = $#lefts;

Then compare them:

if ($number_of_rights != $number_of_lefts)
{
## scream here!!
}

-----Original Message-----
From: S.A. Birl [mailto:sbirl+PERL@concept.temple.edu]
Sent: Wednesday, October 27, 2004 3:23 PM
To: beginners@perl.org
Subject: Recursively counting a matching pattern on a single line.


Given a bookmark:

<DT><A HREF="http://www.perl.com/CPAN-local/doc/FAQs/FAQ/PerlFAQ.html"
ADD_DATE="897592292" LAST_VISIT="982769648" LAST_MODIFIED="982769648"
ID="rdf:#$rsy5Z">PERL FAQ</A>


Wondering if it's possible to have 2 counters that would keep track of the
number of < and > encountered.

Im looking to see if there's a balance of < and >, and scream as soon as
there isnt a balance.

So given the above bookmark, I know that there's 3 <  and  3 >
and the pattern is basically <><><> and not <<>> or >><, etc.

Thanks
Birl

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On Oct 27, brian.barto@spectrum-health.org (nospam-brian.barto@spectrum-hea.
.:

Brian:  Assign each line to the $_ variable and try this to get the number o
f
Brian:  instances of < and >
Brian:
Brian:  The match operator works on $_ by default. If you match on < or > gl
obally
Brian:  and assign it to an array like so:
Brian:
Brian:  @rights = m/\</g;
Brian:  @lefts = m/\>/g;
Brian:
Brian:  You are left with arrays that contain all the matches. Now all you n
eed to
Brian:  do is count the number of elements in each array like so:
Brian:
Brian:  $number_of_rights = $#rights;
Brian:  $number_of_lefts = $#lefts;
Brian:
Brian:  Then compare them:
Brian:
Brian:  if ($number_of_rights != $number_of_lefts)
Brian:  {
Brian:  	## scream here!!
Brian:  }



Great!  That works.  But I was looking to get a little trickier with it.


If I had:
<DT <A HREF="http://www.perl.com/CPAN-local/doc/FAQs/FAQ/PerlFAQ.html" ADD_D
ATE="897592292" LAST_VISIT="982769648" LAST_MODIFIED="982769648" ID="rdf:#$r
sy5Z">PERL FAQ</A>

I wanted it to scream when it reached the 2nd <  ; stopping the pattern
matching from continuing any further.  Is that possible, or would
something more complex be needed?

If it's too complex, I'll just with what I have for now.


Thanks
Birl

Please do not CC me responses to my own posts.
I'll read the responses on the list.

Great!  That works.  But I was looking to get a little trickier with it.


----------------

If you want to make sure they are alternating like <><><> etc... I would do
this:

$_ = $line;

@syms = m/[<>]/g;
$string = join("", @syms);
if ($strings !~ m/^<(>< )*>$/)
{
## Scream here!
}

The regular expression:

m/^<(>< )*>$/

will ensure that it starts with < and ends with > and anything in between
will be "><" which I think should do the trick. That logic is pretty hairy
though and I could be missing something.

Correction: "$strings" in the if statement should be "$string"

-----Original Message-----
From: brian.barto@spectrum-health.org
[mailto:brian.barto@spectrum-health.org]
Sent: Wednesday, October 27, 2004 4:04 PM
To: beginners@perl.org
Subject: RE: Recursively counting a matching pattern on a single line.


Great!  That works.  But I was looking to get a little trickier with it.


----------------

If you want to make sure they are alternating like <><><> etc... I would do
this:

$_ = $line;

@syms = m/[<>]/g;
$string = join("", @syms);
if ($strings !~ m/^<(>< )*>$/)
{
## Scream here!
}

The regular expression:

m/^<(>< )*>$/

will ensure that it starts with < and ends with > and anything in between
will be "><" which I think should do the trick. That logic is pretty hairy
though and I could be missing something.

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On Oct 27, brian.barto@spectrum-health.org (nospam-brian.barto@spectrum-hea.
.:

Brian:
Brian:  If you want to make sure they are alternating like <><><> etc... I w
ould do
Brian:  this:
Brian:
Brian:  $_ = $line;
Brian:
Brian:  @syms = m/[<>]/g;
Brian:  $string = join("", @syms);
Brian:  if ($strings !~ m/^<(>< )*>$/)
Brian:  {
Brian:  	## Scream here!
Brian:  }
Brian:
Brian:  The regular expression:
Brian:
Brian:  m/^<(>< )*>$/
Brian:
Brian:  will ensure that it starts with < and ends with > and anything in be
tween
Brian:  will be "><" which I think should do the trick. That logic is pretty
 hairy
Brian:  though and I could be missing something.



Wouldnt m/[<>]/g literally match <> and not <characters>?

Why wouldnt it be m/[<.+>]/g ?



Thanks
Birl

Please do not CC me responses to my own posts.
I'll read the responses on the list.

Wouldnt m/[<>]/g literally match <> and not <characters>?

Why wouldnt it be m/[<.+>]/g ?



Thanks
Birl

-----------

Someone correct me if I'm wrong but putting characters inside brackets []
defines a character class. Or a group of characters you want to match on,
not necessarily in that order.

For instance m/[0-9]/g matches on any single digit number, not "0123456789"

S.A. Birl wrote:
> Wondering if it's possible to have 2 counters that would keep track
> of the number of < and > encountered.
>
> Im looking to see if there's a balance of < and >, and scream as soon
> as there isnt a balance.
>
> So given the above bookmark, I know that there's 3 <  and  3 > and
> the pattern is basically <><><> and not <<>> or >><, etc.

Assuming that you are actually not interested in *counting* them, this
is one approach:

print "Oops!\n" unless $string =~ /^[^<>]*(?:(?:<[^<>]*> )*[^<>]*)*$/;

--
Gunnar Hjalmarsson
Email: http://www.gunnar.cc/cgi-bin/contact.pl

brian.barto@spectrum-health.org wrote:
> Assign each line to the $_ variable and try this to get the number of
> instances of < and >
>
> The match operator works on $_ by default. If you match on < or > globally
> and assign it to an array like so:
>
> @rights = m/\</g;
> @lefts = m/\>/g;

See the FAQ for a more efficient way to do this.

perldoc -q "How can I count the number of occurrences of a substring within 
a
string"


> You are left with arrays that contain all the matches. Now all you need to
> do is count the number of elements in each array like so:
>
> $number_of_rights = $#rights;
> $number_of_lefts = $#lefts;

An array in scalar context returns the number of elements in that array.  Yo
u
are using the index of the last element of the array which *may* (or may not
!)
be one less than the number of elements.


John
--
use Perl;
program
fulfillment