[24583] in Perl-Users-Digest
Perl-Users Digest, Issue: 6759 Volume: 10
daemon@ATHENA.MIT.EDU (Perl-Users Digest)
Fri Jul 2 06:05:49 2004
Date: Fri, 2 Jul 2004 03:05:05 -0700 (PDT)
From: Perl-Users Digest <Perl-Users-Request@ruby.OCE.ORST.EDU>
To: Perl-Users@ruby.OCE.ORST.EDU (Perl-Users Digest)
Perl-Users Digest Fri, 2 Jul 2004 Volume: 10 Number: 6759
Today's topics:
Counting $var characters <nospam@xx.com>
Re: Counting $var characters <nospam@xx.com>
Re: Counting $var characters <noreply@gunnar.cc>
Re: Counting $var characters <nobull@mail.com>
Re: Counting $var characters <nobull@mail.com>
Re: Counting $var characters <krahnj@acm.org>
Re: Counting $var characters <noreply@gunnar.cc>
Re: Getting to variables contained in a typeglob refere <nobull@mail.com>
Re: Incorrect file download problem. <nobull@mail.com>
Posting Guidelines for comp.lang.perl.misc ($Revision: tadmc@augustmail.com
Re: Q: re the kind of reference to be bless()ed in OO p <nobull@mail.com>
Re: Q: re the kind of reference to be bless()ed in OO p (Anno Siegel)
Re: Request for comments on a JPEG metadata Perl module (Martin Herrmann)
Re: Request for comments on a JPEG metadata Perl module (Martin Herrmann)
Re: shouldnt this evaluate in a scalar context??? (Anno Siegel)
Re: shouldnt this evaluate in a scalar context??? <noreply@gunnar.cc>
Re: shouldnt this evaluate in a scalar context??? <krahnj@acm.org>
Digest Administrivia (Last modified: 6 Apr 01) (Perl-Users-Digest Admin)
----------------------------------------------------------------------
Date: Fri, 02 Jul 2004 00:33:37 -0700
From: Eric <nospam@xx.com>
Subject: Counting $var characters
Message-Id: <40E50FD1.83BE46AD@xx.com>
Maybe it's the wee hour of the morning or maybe I missed it. I've
looked up Count & Total in 3 Perl books but am still baffled. It should
be so easy.
RE:
$line =~ m/(.*),(.*)/;
$var1 = $1;
$var2 = $2;
$var1count = ???
$var2count = ???
I am pulling 2 $variables out of each $line read in from a .csv file.
Each $var will contain some or all of the following:
1. A-Z characters (\w)
2. 0-9 digits (\d)
3. A "/" (forward slash) (/)
What is needed to is the total number of characters, digits and "/" (if
it exists) for further processing.
For example, if $var1 = BYX13/MM ... the total count would be 8. If
$var2 = 84R/QZ7 ... the total count would be 7.
There must be a simple command to do this but I'm not finding it. Any
help is appreciated.
Thank you.
E.
------------------------------
Date: Fri, 02 Jul 2004 01:11:32 -0700
From: Eric <nospam@xx.com>
Subject: Re: Counting $var characters
Message-Id: <40E518B4.EC738F57@xx.com>
Further digging indicates it may possibly be the length() function. Go
figure. I had 'Count' and 'Total' too much in mind.
E.
------------------------------
Date: Fri, 02 Jul 2004 10:23:36 +0200
From: Gunnar Hjalmarsson <noreply@gunnar.cc>
Subject: Re: Counting $var characters
Message-Id: <2kkke4F3fqbhU1@uni-berlin.de>
Eric wrote:
>
> $line =~ m/(.*),(.*)/;
> $var1 = $1;
> $var2 = $2;
> $var1count = ???
> $var2count = ???
>
> I am pulling 2 $variables out of each $line read in from a .csv
> file. Each $var will contain some or all of the following:
>
> 1. A-Z characters (\w)
That's not what the character class \w represents. Look it up in
"perldoc perlre".
> 2. 0-9 digits (\d)
> 3. A "/" (forward slash) (/)
>
> What is needed to is the total number of characters, digits and "/"
> (if it exists) for further processing.
>
> For example, if $var1 = BYX13/MM ... the total count would be 8.
> If $var2 = 84R/QZ7 ... the total count would be 7.
To simply count *all* characters, check out "perldoc -f length".
If the variables may contain other characters but those you want to
count, you can do:
my $var1count = $var1 =~ tr![A-Z][a-z][0-9]/!!;
Read about the tr/// operator in "perldoc perlop".
--
Gunnar Hjalmarsson
Email: http://www.gunnar.cc/cgi-bin/contact.pl
------------------------------
Date: 02 Jul 2004 09:38:07 +0100
From: Brian McCauley <nobull@mail.com>
Subject: Re: Counting $var characters
Message-Id: <u9lli2olcg.fsf@wcl-l.bham.ac.uk>
Eric <nospam@xx.com> writes:
> What is needed to is the total number of characters, digits and "/" (if
> it exists) for further processing.
>
> For example, if $var1 = BYX13/MM ... the total count would be 8. If
> $var2 = 84R/QZ7 ... the total count would be 7.
To get the length of a string use the length() function.
To count the number of occurrences in a string of characters in a
specific set use tr/// as described in the FAQ "How can I count the
number of occurrences of a substring within a string?". Note the FAQ
implies tr/// will count occurrences of only a single characters but
actually that's not right.
--
\\ ( )
. _\\__[oo
.__/ \\ /\@
. l___\\
# ll l\\
###LL LL\\
------------------------------
Date: 02 Jul 2004 09:41:37 +0100
From: Brian McCauley <nobull@mail.com>
Subject: Re: Counting $var characters
Message-Id: <u9hdsqol6m.fsf@wcl-l.bham.ac.uk>
Eric <nospam@xx.com> writes:
> $line =~ m/(.*),(.*)/;
> $var1 = $1;
> $var2 = $2;
This is more simply written
($var1, $var2) = $line =~ m/(.*),(.*)/;
You should always declare all variables as lexically scoped in the
smallest applicable lexical scope unless you have a positive reason to
do otherwise. BTW: this is not perculliar to Perl, it applies in all
programming languges - allowing that a language not having lexical
variables is a positive reason :-).
For Perl this means that most of the time the declaration of scalars
should be combined with the first assignment. BTW: this to is not
perculliar to Perl, it also applies in other programming languges
where assignment and declaration can be combined.
By following this convention you will be able to get maximum beniefit
out of putting "use strict" at the top of all your scripts.
Try to get into this habit now, do not wait for your failure to do so
to cause you the unecessary distress of wasting your own time and that
of other people. The longer you leave it the harder you will find it
to adjust. Worse still, if you leave it too long you may never adjust
and may mutate into a bitter and twisted troll.
So in practice one would usually write:
my ($var1, $var2) = $line =~ m/(.*),(.*)/;
Finally, for simply splitting on a delimiter it would be possibly more
idiomatic to use split() rather than m//.
--
\\ ( )
. _\\__[oo
.__/ \\ /\@
. l___\\
# ll l\\
###LL LL\\
------------------------------
Date: Fri, 02 Jul 2004 09:42:13 GMT
From: "John W. Krahn" <krahnj@acm.org>
Subject: Re: Counting $var characters
Message-Id: <40E52DDD.1419AD8@acm.org>
Gunnar Hjalmarsson wrote:
>
> Eric wrote:
> >
> > $line =~ m/(.*),(.*)/;
> > $var1 = $1;
> > $var2 = $2;
> > $var1count = ???
> > $var2count = ???
> >
> > I am pulling 2 $variables out of each $line read in from a .csv
> > file. Each $var will contain some or all of the following:
> >
> > 1. A-Z characters (\w)
>
> That's not what the character class \w represents. Look it up in
> "perldoc perlre".
>
> > 2. 0-9 digits (\d)
> > 3. A "/" (forward slash) (/)
> >
> > What is needed to is the total number of characters, digits and "/"
> > (if it exists) for further processing.
> >
> > For example, if $var1 = BYX13/MM ... the total count would be 8.
> > If $var2 = 84R/QZ7 ... the total count would be 7.
>
> To simply count *all* characters, check out "perldoc -f length".
>
> If the variables may contain other characters but those you want to
> count, you can do:
>
> my $var1count = $var1 =~ tr![A-Z][a-z][0-9]/!!;
The OP said that he only wanted to count 'total number of characters,
digits and "/"', he didn't say that he wanted to count '[' and ']' as
well.
> Read about the tr/// operator in "perldoc perlop".
Yes, please do.
John
--
use Perl;
program
fulfillment
------------------------------
Date: Fri, 02 Jul 2004 11:53:12 +0200
From: Gunnar Hjalmarsson <noreply@gunnar.cc>
Subject: Re: Counting $var characters
Message-Id: <2kkpmnF3g22oU1@uni-berlin.de>
John W. Krahn wrote:
> Gunnar Hjalmarsson wrote:
>> If the variables may contain other characters but those you want
>> to count, you can do:
>>
>> my $var1count = $var1 =~ tr![A-Z][a-z][0-9]/!!;
>
> The OP said that he only wanted to count 'total number of
> characters, digits and "/"', he didn't say that he wanted to count
> '[' and ']' as well.
Hmm.. I obviously meant to suggest:
my $var1count = $var1 =~ tr!A-Za-z0-9/!!;
Thanks.
--
Gunnar Hjalmarsson
Email: http://www.gunnar.cc/cgi-bin/contact.pl
------------------------------
Date: 02 Jul 2004 10:29:36 +0100
From: Brian McCauley <nobull@mail.com>
Subject: Re: Getting to variables contained in a typeglob referenced by a scalar.
Message-Id: <u98ye2oiyn.fsf@wcl-l.bham.ac.uk>
ddtl <this.is@invalid> writes:
> >
> >If you want the array and hash in the GLOB, then you write:
> >@$$fh and %$$fh respectively (not @$fh and not %$fh);
> >
> >It 'works' because the deference syntax is unambiguous.
>
> But why not use $@$fh or $%$fh - it shouldn't be ambiguous -
> the first $ indicates that fh is a scalar.
That's first counting from the right or from the leaves of the
parse tree. While in execution order the leafmost part of the
parse tree is usually first most humans when reading a L2R rendered
language will read "first" as meaning leftmost.
> The second funny character
> indicates that we dereference scalar and getting to the typeglob
If you wanted to unambiguously get to a typeglob it would be a *.
However there's a grey area between GLOBs and SCALARs - this is for
historical reasons. Just like references can be handled as scalars so
can GLOBs. So for this reason you can use $ even though I'd always
use * for the sake of clarity. You cannot use a @ or a % because $foo
is not usable as a hashref or an arrayref.
my $foo = \*bar; # $foo contains a GLOBref
# Try to dereference $foo as a GLOB
print *$foo; # prints *main::bar
print ref \*$foo; # prints GLOB
# Try to dereference $foo as a scalar but actually get a GLOB
print ref \$$foo; # prints GLOB
# Try to dereference $foo as an array
print ref \@$foo; # error - Not an ARRAY reference
> (typeglob is either a scalar, or array or hash, so every funny
> character should do - why give special privilege to $?).
No. A reference is a special type of scalar value that points to a
single thingy. A (fake) glob is a special type of scalar value that
points to one thingy of each type.
If $foo was a reference to a reference to a hash you'd use %{${$foo}}
or just %$$foo.
If $foo was a reference to a glob to and you want the hash part of the
glob I'd use %{*{$foo}} but because GLOBs can sometimes be treated as
SCALAR you can still just use %$$foo.
--
\\ ( )
. _\\__[oo
.__/ \\ /\@
. l___\\
# ll l\\
###LL LL\\
------------------------------
Date: 02 Jul 2004 10:34:51 +0100
From: Brian McCauley <nobull@mail.com>
Subject: Re: Incorrect file download problem.
Message-Id: <u94qoqoipw.fsf@wcl-l.bham.ac.uk>
qakamran@yahoo.com (Sphinx) writes:
> On my website, I am trying to use following perl script to download an
> .exe file from server. This script downloads a smaller file then the
> original one, (leaving nearly 100 bytes). I tried to compare the
> contents (binary) of both files and they are same.
It is impossible for two files of different sizes to have identical
content. What did you really mean to say?
> I think I am using correct mime type; anyone has any idea, what could
> be the problem?
You forgot to open the file in binary mode. (perldoc -f binmode).
--
\\ ( )
. _\\__[oo
.__/ \\ /\@
. l___\\
# ll l\\
###LL LL\\
------------------------------
Date: Fri, 02 Jul 2004 02:22:19 -0500
From: tadmc@augustmail.com
Subject: Posting Guidelines for comp.lang.perl.misc ($Revision: 1.5 $)
Message-Id: <9pGdnYjuZ8-2kHjdRVn-jA@august.net>
Outline
Before posting to comp.lang.perl.misc
Must
- Check the Perl Frequently Asked Questions (FAQ)
- Check the other standard Perl docs (*.pod)
Really Really Should
- Lurk for a while before posting
- Search a Usenet archive
If You Like
- Check Other Resources
Posting to comp.lang.perl.misc
Is there a better place to ask your question?
- Question should be about Perl, not about the application area
How to participate (post) in the clpmisc community
- Carefully choose the contents of your Subject header
- Use an effective followup style
- Speak Perl rather than English, when possible
- Ask perl to help you
- Do not re-type Perl code
- Provide enough information
- Do not provide too much information
- Do not post binaries, HTML, or MIME
Social faux pas to avoid
- Asking a Frequently Asked Question
- Asking a question easily answered by a cursory doc search
- Asking for emailed answers
- Beware of saying "doesn't work"
- Sending a "stealth" Cc copy
Be extra cautious when you get upset
- Count to ten before composing a followup when you are upset
- Count to ten after composing and before posting when you are upset
-----------------------------------------------------------------
Posting Guidelines for comp.lang.perl.misc ($Revision: 1.5 $)
This newsgroup, commonly called clpmisc, is a technical newsgroup
intended to be used for discussion of Perl related issues (except job
postings), whether it be comments or questions.
As you would expect, clpmisc discussions are usually very technical in
nature and there are conventions for conduct in technical newsgroups
going somewhat beyond those in non-technical newsgroups.
The article at:
http://www.catb.org/~esr/faqs/smart-questions.html
describes how to get answers from technical people in general.
This article describes things that you should, and should not, do to
increase your chances of getting an answer to your Perl question. It is
available in POD, HTML and plain text formats at:
http://mail.augustmail.com/~tadmc/clpmisc.shtml
For more information about netiquette in general, see the "Netiquette
Guidelines" at:
http://andrew2.andrew.cmu.edu/rfc/rfc1855.html
A note to newsgroup "regulars":
Do not use these guidelines as a "license to flame" or other
meanness. It is possible that a poster is unaware of things
discussed here. Give them the benefit of the doubt, and just
help them learn how to post, rather than assume
A note about technical terms used here:
In this document, we use words like "must" and "should" as
they're used in technical conversation (such as you will
encounter in this newsgroup). When we say that you *must* do
something, we mean that if you don't do that something, then
it's unlikely that you will benefit much from this group.
We're not bossing you around; we're making the point without
lots of words.
Do *NOT* send email to the maintainer of these guidelines. It will be
discarded unread. The guidelines belong to the newsgroup so all
discussion should appear in the newsgroup. I am just the secretary that
writes down the consensus of the group.
Before posting to comp.lang.perl.misc
Must
This section describes things that you *must* do before posting to
clpmisc, in order to maximize your chances of getting meaningful replies
to your inquiry and to avoid getting flamed for being lazy and trying to
have others do your work.
The perl distribution includes documentation that is copied to your hard
drive when you install perl. Also installed is a program for looking
things up in that (and other) documentation named 'perldoc'.
You should either find out where the docs got installed on your system,
or use perldoc to find them for you. Type "perldoc perldoc" to learn how
to use perldoc itself. Type "perldoc perl" to start reading Perl's
standard documentation.
Check the Perl Frequently Asked Questions (FAQ)
Checking the FAQ before posting is required in Big 8 newsgroups in
general, there is nothing clpmisc-specific about this requirement.
You are expected to do this in nearly all newsgroups.
You can use the "-q" switch with perldoc to do a word search of the
questions in the Perl FAQs.
Check the other standard Perl docs (*.pod)
The perl distribution comes with much more documentation than is
available for most other newsgroups, so in clpmisc you should also
see if you can find an answer in the other (non-FAQ) standard docs
before posting.
It is *not* required, or even expected, that you actually *read* all of
Perl's standard docs, only that you spend a few minutes searching them
before posting.
Try doing a word-search in the standard docs for some words/phrases
taken from your problem statement or from your very carefully worded
"Subject:" header.
Really Really Should
This section describes things that you *really should* do before posting
to clpmisc.
Lurk for a while before posting
This is very important and expected in all newsgroups. Lurking means
to monitor a newsgroup for a period to become familiar with local
customs. Each newsgroup has specific customs and rituals. Knowing
these before you participate will help avoid embarrassing social
situations. Consider yourself to be a foreigner at first!
Search a Usenet archive
There are tens of thousands of Perl programmers. It is very likely
that your question has already been asked (and answered). See if you
can find where it has already been answered.
One such searchable archive is:
http://groups.google.com/advanced_group_search
If You Like
This section describes things that you *can* do before posting to
clpmisc.
Check Other Resources
You may want to check in books or on web sites to see if you can
find the answer to your question.
But you need to consider the source of such information: there are a
lot of very poor Perl books and web sites, and several good ones
too, of course.
Posting to comp.lang.perl.misc
There can be 200 messages in clpmisc in a single day. Nobody is going to
read every article. They must decide somehow which articles they are
going to read, and which they will skip.
Your post is in competition with 199 other posts. You need to "win"
before a person who can help you will even read your question.
These sections describe how you can help keep your article from being
one of the "skipped" ones.
Is there a better place to ask your question?
Question should be about Perl, not about the application area
It can be difficult to separate out where your problem really is,
but you should make a conscious effort to post to the most
applicable newsgroup. That is, after all, where you are the most
likely to find the people who know how to answer your question.
Being able to "partition" a problem is an essential skill for
effectively troubleshooting programming problems. If you don't get
that right, you end up looking for answers in the wrong places.
It should be understood that you may not know that the root of your
problem is not Perl-related (the two most frequent ones are CGI and
Operating System related), so off-topic postings will happen from
time to time. Be gracious when someone helps you find a better place
to ask your question by pointing you to a more applicable newsgroup.
How to participate (post) in the clpmisc community
Carefully choose the contents of your Subject header
You have 40 precious characters of Subject to win out and be one of
the posts that gets read. Don't waste them. Take care while
composing them, they are the key that opens the door to getting an
answer.
Spend them indicating what aspect of Perl others will find if they
should decide to read your article.
Do not spend them indicating "experience level" (guru, newbie...).
Do not spend them pleading (please read, urgent, help!...).
Do not spend them on non-Subjects (Perl question, one-word
Subject...)
For more information on choosing a Subject see "Choosing Good
Subject Lines":
http://www.cpan.org/authors/id/D/DM/DMR/subjects.post
Part of the beauty of newsgroup dynamics, is that you can contribute
to the community with your very first post! If your choice of
Subject leads a fellow Perler to find the thread you are starting,
then even asking a question helps us all.
Use an effective followup style
When composing a followup, quote only enough text to establish the
context for the comments that you will add. Always indicate who
wrote the quoted material. Never quote an entire article. Never
quote a .signature (unless that is what you are commenting on).
Intersperse your comments *following* each section of quoted text to
which they relate. Unappreciated followup styles are referred to as
"top-posting", "Jeopardy" (because the answer comes before the
question), or "TOFU" (Text Over, Fullquote Under).
Reversing the chronology of the dialog makes it much harder to
understand (some folks won't even read it if written in that style).
For more information on quoting style, see:
http://web.presby.edu/~nnqadmin/nnq/nquote.html
Speak Perl rather than English, when possible
Perl is much more precise than natural language. Saying it in Perl
instead will avoid misunderstanding your question or problem.
Do not say: I have variable with "foo\tbar" in it.
Instead say: I have $var = "foo\tbar", or I have $var = 'foo\tbar',
or I have $var = <DATA> (and show the data line).
Ask perl to help you
You can ask perl itself to help you find common programming mistakes
by doing two things: enable warnings (perldoc warnings) and enable
"strict"ures (perldoc strict).
You should not bother the hundreds/thousands of readers of the
newsgroup without first seeing if a machine can help you find your
problem. It is demeaning to be asked to do the work of a machine. It
will annoy the readers of your article.
You can look up any of the messages that perl might issue to find
out what the message means and how to resolve the potential mistake
(perldoc perldiag). If you would like perl to look them up for you,
you can put "use diagnostics;" near the top of your program.
Do not re-type Perl code
Use copy/paste or your editor's "import" function rather than
attempting to type in your code. If you make a typo you will get
followups about your typos instead of about the question you are
trying to get answered.
Provide enough information
If you do the things in this item, you will have an Extremely Good
chance of getting people to try and help you with your problem!
These features are a really big bonus toward your question winning
out over all of the other posts that you are competing with.
First make a short (less than 20-30 lines) and *complete* program
that illustrates the problem you are having. People should be able
to run your program by copy/pasting the code from your article. (You
will find that doing this step very often reveals your problem
directly. Leading to an answer much more quickly and reliably than
posting to Usenet.)
Describe *precisely* the input to your program. Also provide example
input data for your program. If you need to show file input, use the
__DATA__ token (perldata.pod) to provide the file contents inside of
your Perl program.
Show the output (including the verbatim text of any messages) of
your program.
Describe how you want the output to be different from what you are
getting.
If you have no idea at all of how to code up your situation, be sure
to at least describe the 2 things that you *do* know: input and
desired output.
Do not provide too much information
Do not just post your entire program for debugging. Most especially
do not post someone *else's* entire program.
Do not post binaries, HTML, or MIME
clpmisc is a text only newsgroup. If you have images or binaries
that explain your question, put them in a publically accessible
place (like a Web server) and provide a pointer to that location. If
you include code, cut and paste it directly in the message body.
Don't attach anything to the message. Don't post vcards or HTML.
Many people (and even some Usenet servers) will automatically filter
out such messages. Many people will not be able to easily read your
post. Plain text is something everyone can read.
Social faux pas to avoid
The first two below are symptoms of lots of FAQ asking here in clpmisc.
It happens so often that folks will assume that it is happening yet
again. If you have looked but not found, or found but didn't understand
the docs, say so in your article.
Asking a Frequently Asked Question
It should be understood that you may have missed the applicable FAQ
when you checked, which is not a big deal. But if the Frequently
Asked Question is worded similar to your question, folks will assume
that you did not look at all. Don't become indignant at pointers to
the FAQ, particularly if it solves your problem.
Asking a question easily answered by a cursory doc search
If folks think you have not even tried the obvious step of reading
the docs applicable to your problem, they are likely to become
annoyed.
If you are flamed for not checking when you *did* check, then just
shrug it off (and take the answer that you got).
Asking for emailed answers
Emailed answers benefit one person. Posted answers benefit the
entire community. If folks can take the time to answer your
question, then you can take the time to go get the answer in the
same place where you asked the question.
It is OK to ask for a *copy* of the answer to be emailed, but many
will ignore such requests anyway. If you munge your address, you
should never expect (or ask) to get email in response to a Usenet
post.
Ask the question here, get the answer here (maybe).
Beware of saying "doesn't work"
This is a "red flag" phrase. If you find yourself writing that,
pause and see if you can't describe what is not working without
saying "doesn't work". That is, describe how it is not what you
want.
Sending a "stealth" Cc copy
A "stealth Cc" is when you both email and post a reply without
indicating *in the body* that you are doing so.
Be extra cautious when you get upset
Count to ten before composing a followup when you are upset
This is recommended in all Usenet newsgroups. Here in clpmisc, most
flaming sub-threads are not about any feature of Perl at all! They
are most often for what was seen as a breach of netiquette. If you
have lurked for a bit, then you will know what is expected and won't
make such posts in the first place.
But if you get upset, wait a while before writing your followup. I
recommend waiting at least 30 minutes.
Count to ten after composing and before posting when you are upset
After you have written your followup, wait *another* 30 minutes
before committing yourself by posting it. You cannot take it back
once it has been said.
AUTHOR
Tad McClellan <tadmc@augustmail.com> and many others on the
comp.lang.perl.misc newsgroup.
------------------------------
Date: 02 Jul 2004 10:02:59 +0100
From: Brian McCauley <nobull@mail.com>
Subject: Re: Q: re the kind of reference to be bless()ed in OO perl
Message-Id: <u9d63eok70.fsf@wcl-l.bham.ac.uk>
Michele Dondi <bik.mido@tiscalinet.it> writes:
> I must say in advance that I know the most basic aspects of OO
> programmin in Perl, but I've never been really proficient with it. So
> pardon my ignorance...
>
> It is often said that the reference that gets bless()ed to create an
> object is one to an anonymous hash, which makes perfectly sense, as
> the examples abunding everywhere clearly indicate.
>
> I've also seen bless()ing something like an empty {array,hash}ref to
> be used "solely" as an index into a package/class global hash to
> enforce some degree of encapsulation.
>
> Now I wonder if there are common/cool/smart/witty cases in which it is
> natural to use a reference to some other kind of object, like e.g. a
> sub, or filehandle, etc.
The obvious example of the filehandle (or actually the GLOB) is
the IO:: classes.
The other time you'd need to use something other than an ARRAY or HASH
is when you want an object that implements an array and hash API. For
this you need to overload the @{} and %{} operators to return (often
tied) array and hashes. You can't sensibly define the %{} operator if
the object is itself a blessed hash.
--
\\ ( )
. _\\__[oo
.__/ \\ /\@
. l___\\
# ll l\\
###LL LL\\
------------------------------
Date: 2 Jul 2004 09:50:22 GMT
From: anno4000@lublin.zrz.tu-berlin.de (Anno Siegel)
Subject: Re: Q: re the kind of reference to be bless()ed in OO perl
Message-Id: <cc3b4u$jcg$1@mamenchi.zrz.TU-Berlin.DE>
Brian McCauley <nobull@mail.com> wrote in comp.lang.perl.misc:
> Michele Dondi <bik.mido@tiscalinet.it> writes:
[...]
> > Now I wonder if there are common/cool/smart/witty cases in which it is
> > natural to use a reference to some other kind of object, like e.g. a
> > sub, or filehandle, etc.
>
> The obvious example of the filehandle (or actually the GLOB) is
> the IO:: classes.
>
> The other time you'd need to use something other than an ARRAY or HASH
> is when you want an object that implements an array and hash API. For
> this you need to overload the @{} and %{} operators to return (often
> tied) array and hashes. You can't sensibly define the %{} operator if
> the object is itself a blessed hash.
Well, you can, for relaxed values of "sensibly".
Say you want an object that looks like a hashref to the user, but counts
accesses in an internal field:
package MyClass;
sub new {
my $class = shift;
bless {
count => 0,
content => { @_ },
}, $class;
}
The user should only see the "content" hash, but other methods must
be able to access the full object. The trick is to make the overloading
accessor caller dependent:
sub as_hash {
my $ob = shift;
return $ob if caller eq __PACKAGE__;
$ob->{ count} ++;
return $ob->{ content};
}
use overload ( '%{}' => 'as_hash' );
A method like
sub count { $_[ 0]->{ count} }
can still access the "outer" hash because its caller is MyClass.
Anno
------------------------------
Date: 2 Jul 2004 02:44:31 -0700
From: mrh1@gmx.net (Martin Herrmann)
Subject: Re: Request for comments on a JPEG metadata Perl module
Message-Id: <350918ef.0407020144.78b0c480@posting.google.com>
google@milbaugh.com (GreenLight) wrote in message news:<c4b60ce1.0407010534.72248bfd@posting.google.com>...
> Stefano Bettelli <stefano_bettelli@yahoo.fr> wrote in message news:<pan.2004.06.26.17.57.26.885135@yahoo.fr>...
> > Hi,
> >
> > I got recently interested in the possibility of designing a Perl
> > library for reading and modifying JPEG image metadata (with Exif
> > info, IPTC info, comments, thumbnails and so on). This kind of
> > additional data stored in the image itself is very useful for
> > organising digital photo collections. For various reasons, the
> > existing Perl libraries and programs do not fully satisfy me,
> > so I decided to enter the arena and write a Perl module (this is
> > also a good way to learn the language better ...).
>
> I have been wanting to create a catalog of my photos for quite some
> time. I have thousands of photos that I have taken over the past five
> years, and it looks like your module could help me quite a bit.
>
> > 4) Do you have any idea on how it could be extended? Whether
> > there are interesting functionalities I did not think about?
>
> I guess that I should read the information regarding the JPEG format
> to get the answer, but maybe you know this: would it be possible to
> add segments of information to the file that were of my own design? I
> would like to add some flags to each photo that would show that I have
> completed cataloging it.
That flag is already there:
I would use the IPTC/edit status for this purpose.
> This is just what I need: the date & time of the photo, etc. I can use
> this info to stick a record in a database that holds basic info & the
> filesystem location of the photo. I would like to be able to set some
> kind of flag in the file, then, so that when I did a subsequent sweep
> of the disk for image files, I could easily skip photos that had
> already been processed.
If you are looking for a GUI to manage your photos, you may have a
look at Mapivi (http://mapivi.de.vu), it's free, runs on Windows and
UNIX and the next version will use Stefanos Bettellis new module
Image::MetaInfo::JPEG.
Bye,
Martin
------------------------------
Date: 2 Jul 2004 02:53:33 -0700
From: mrh1@gmx.net (Martin Herrmann)
Subject: Re: Request for comments on a JPEG metadata Perl module
Message-Id: <350918ef.0407020153.4423806a@posting.google.com>
Josef Moellers <josef.moellers@fujitsu-siemens.com> wrote in message news:<cc16et$oc5$1@nntp.fujitsu-siemens.com>...
> GreenLight wrote:
> > Stefano Bettelli <stefano bettelli@yahoo.fr> wrote in message news:<pan
> .2004.06.26.17.57.26.885135@yahoo.fr>...
>
> > I guess that I should read the information regarding the JPEG format
> > to get the answer, but maybe you know this: would it be possible to
> > add segments of information to the file that were of my own design? I
> > would like to add some flags to each photo that would show that I have
> > completed cataloging it.
>
> It depends. Obviously the standard is designed such that any application
>
> can skip those tags that it doesn't know. However, you cannot guarantee
> that all software is written properly.
I'm sure that this approach will cause nothing but trouble. There are
so many picture applications which e.g. will only handle the first
comment segment and throw away the rest ...
As noted in the other post, I strongly recomment using "standard"
segements for storing such informations, like the IPTC info
(http://www.iptc.org).
> > I used you module to parse a file from my camera (Casio QV-2000UX).
> > Here is part of the info that was returned:
> >
> [ ... ]
> >
> > This is just what I need: the date & time of the photo, etc. I can use
> > this info to stick a record in a database that holds basic info & the
> > filesystem location of the photo. I would like to be able to set some
> > kind of flag in the file, then, so that when I did a subsequent sweep
> > of the disk for image files, I could easily skip photos that had
> > already been processed.
>
> I, too, did some work on this subject.
> I have a Kodac DC240 which stores the images in EXIF format.
>
> I store all my photos in the "Exif" directory.
> Also, there exist "Photo", "Info" and "Thumb" directories.
> When I scan the photos, I scan the Exif directory (File::Find), then, if
>
> no entry exists in Photo, I extract the large image, if no entry exists
> in Info, I extract the information, if no entry exists in Thumb, I
> extract the thumbnail.
> Then I have .alb files which describe what photos belong together and I
What are .alb files?
> create html pages with the thumbnails that have links to the large images
> .
I'm not exacly sure, that I understand everything you wrote, but it
seems to me, that most of this (including the html export, handling of
EXIF infos and thumbnails) can be done with Mapivi
(http://mapivi.de.vu).
Bye,
Martin
------------------------------
Date: 2 Jul 2004 08:38:28 GMT
From: anno4000@lublin.zrz.tu-berlin.de (Anno Siegel)
Subject: Re: shouldnt this evaluate in a scalar context???
Message-Id: <cc36u4$fsk$1@mamenchi.zrz.TU-Berlin.DE>
Gunnar Hjalmarsson <noreply@gunnar.cc> wrote in comp.lang.perl.misc:
> dutone wrote:
> >
> > my %h = (roach=>'raid',slut=>'fun');
> > my @a = qw(roach slut);
> > print "YEA!!" if @h{@a} == @a
> >
> > Since @a is a list and evals to 2 in scalar context...
>
> It does not evaluate to 2 in scalar context since it is a list. It
> does so because it is an array.
>
> > and @h{@a} returns a list, but this list doesnt go scalar.
>
> Yes it does. A list returns the last element in scalar context, and
> that is what happens. Perl would have told you so if you had had
> warnings enabled.
>
> Please use warnings before posting about code that does not output
> what you had expected.
>
> > What I was trying to do is see if all the keys in a array
> > (list,whatever)
>
> The distinction is important in this case!
>
> > Any of you perl thugs have any ideas?
>
> print "YEA!!" if @{ [@h{@a}] } == @a;
@a = qw( gaga gogo gigi);
print "YEA!!" if @{ [@h{@a}] } == @a;
YEA!!
Still not much of a test. The whole idea doesn't seem to work very
well.
Anno
------------------------------
Date: Fri, 02 Jul 2004 11:18:04 +0200
From: Gunnar Hjalmarsson <noreply@gunnar.cc>
Subject: Re: shouldnt this evaluate in a scalar context???
Message-Id: <2kknklF3cjvsU1@uni-berlin.de>
Anno Siegel wrote:
> Gunnar Hjalmarsson wrote:
>>
>> print "YEA!!" if @{ [@h{@a}] } == @a;
>
> @a = qw( gaga gogo gigi);
> print "YEA!!" if @{ [@h{@a}] } == @a;
>
> YEA!!
>
> Still not much of a test. The whole idea doesn't seem to work
> very well.
Well, it keeps you happy by printing "YEA!!" all the time. :)
Besides that, you are right, of course... Thanks for the correction!
How about this as 'the ultimate solution' to this problem:
print "YEA!!" if @a == grep defined, @h{@a};
--
Gunnar Hjalmarsson
Email: http://www.gunnar.cc/cgi-bin/contact.pl
------------------------------
Date: Fri, 02 Jul 2004 09:53:21 GMT
From: "John W. Krahn" <krahnj@acm.org>
Subject: Re: shouldnt this evaluate in a scalar context???
Message-Id: <40E53078.889344EF@acm.org>
Gunnar Hjalmarsson wrote:
>
> Anno Siegel wrote:
> > Gunnar Hjalmarsson wrote:
> >>
> >> print "YEA!!" if @{ [@h{@a}] } == @a;
> >
> > @a = qw( gaga gogo gigi);
> > print "YEA!!" if @{ [@h{@a}] } == @a;
> >
> > YEA!!
> >
> > Still not much of a test. The whole idea doesn't seem to work
> > very well.
>
> Well, it keeps you happy by printing "YEA!!" all the time. :)
>
> Besides that, you are right, of course... Thanks for the correction!
>
> How about this as 'the ultimate solution' to this problem:
>
> print "YEA!!" if @a == grep defined, @h{@a};
That compares the hash keys in @a to the hash values in @h{@a} which is
probably not what the OP wants.
John
--
use Perl;
program
fulfillment
------------------------------
Date: 6 Apr 2001 21:33:47 GMT (Last modified)
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Subject: Digest Administrivia (Last modified: 6 Apr 01)
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