[31288] in Perl-Users-Digest
Perl-Users Digest, Issue: 2533 Volume: 11
daemon@ATHENA.MIT.EDU (Perl-Users Digest)
Sun Aug 2 18:09:44 2009
Date: Sun, 2 Aug 2009 15:09:08 -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 Sun, 2 Aug 2009 Volume: 11 Number: 2533
Today's topics:
Re: FAQ 7.29 How can I use a variable as a variable nam <kst-u@mib.org>
Re: FAQ 7.29 How can I use a variable as a variable nam <uri@stemsystems.com>
open file, delete 4 lines if they meet criteria <fgnowfg@gmail.com>
Re: open file, delete 4 lines if they meet criteria <ben@morrow.me.uk>
Re: open file, delete 4 lines if they meet criteria <fgnowfg@gmail.com>
Re: open file, delete 4 lines if they meet criteria <tadmc@seesig.invalid>
Digest Administrivia (Last modified: 6 Apr 01) (Perl-Users-Digest Admin)
----------------------------------------------------------------------
Date: Sun, 02 Aug 2009 12:47:34 -0700
From: Keith Thompson <kst-u@mib.org>
Subject: Re: FAQ 7.29 How can I use a variable as a variable name?
Message-Id: <ln8wi2q9pl.fsf@nuthaus.mib.org>
PerlFAQ Server <brian@stonehenge.com> writes:
[..]
> 7.29: How can I use a variable as a variable name?
>
> Beginners often think they want to have a variable contain the name of a
> variable.
>
> $fred = 23;
> $varname = "fred";
> ++$$varname; # $fred now 24
>
> This works *sometimes*, but it is a very bad idea for two reasons.
>
> The first reason is that this technique *only works on global
> variables*.
[snip]
This FAQ should probably mention that you *can* have a variable
containing a variable name, if you use eval, like this:
========================================
#!/usr/bin/perl
use strict;
use warnings;
my $foo="hello";
my $variable_name = 'foo';
print eval "\$$variable_name", "\n";
========================================
or even like this:
========================================
#!/usr/bin/perl
use strict;
use warnings;
my $foo="hello";
my $variable_name = '$foo';
print eval($variable_name), "\n";
========================================
It should also mention that it's almost always a bad idea.
--
Keith Thompson (The_Other_Keith) kst-u@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"
------------------------------
Date: Sun, 02 Aug 2009 16:38:38 -0400
From: Uri Guttman <uri@stemsystems.com>
Subject: Re: FAQ 7.29 How can I use a variable as a variable name?
Message-Id: <878wi2rlwx.fsf@quad.sysarch.com>
>>>>> "KT" == Keith Thompson <kst-u@mib.org> writes:
KT> It should also mention that it's almost always a bad idea.
agreed. my edit which could be put in is this:
Using a string for a variable name (via symbolic references or eval)
means you are using the symbol table (which is a specialized hash tree)
for a data structure. This has many flaws including action at a
distance, the inability to isolate your data and it is slower. Instead
you should just use your own data structures composed of hashes and
hash/array references. Read perllol and perldsc for more on how to do
this. The symbol table should only be modified if you are actually
manipulating symbols and not just your data.
(that can be improved upon i am sure. but the gist is pretty good IMO).
and yes, i know that eval can even access lexicals with strings for
names but that is even nastier than accessing globals!
uri
--
Uri Guttman ------ uri@stemsystems.com -------- http://www.sysarch.com --
----- Perl Code Review , Architecture, Development, Training, Support ------
--------- Free Perl Training --- http://perlhunter.com/college.html ---------
--------- Gourmet Hot Cocoa Mix ---- http://bestfriendscocoa.com ---------
------------------------------
Date: Sat, 1 Aug 2009 09:43:33 -0700 (PDT)
From: Faith Greenwood <fgnowfg@gmail.com>
Subject: open file, delete 4 lines if they meet criteria
Message-Id: <fe3ddf44-c8e4-4048-a77d-5e3df11c169f@24g2000yqm.googlegroups.com>
OK, I know how to open a file and delete a specific line, but how can
I delete a section of lines? for instance, I have:
here's the contents of my <FILE>:
the red fox
and the brown fox
quickly and quietly
jumped over
the fence
but for some reason
the fence was
too high
I guess they
will have to try and try again
the farmer saw the goat
and the goat was happy
--end of file
I want my ending file to be as follows:
the red fox
and the brown fox
quickly and quietly
jumped over
the fence
the farmer saw the goat
and the goat was happy
The part I will be deleting will obviously be:
but for some reason
the fence was
too high
I guess they
will have to try and try again
and will alway start with "but for some reason" and end with "will
have to try and try again" but the middle contents will change....how
can I do this in perl?
I have this:
while (<FILE>){
next if /but for some reason/;
last if /will have to try and try again/;
print $_;
}
but this gives me:
the red fox
and the brown fox
quickly and quietly
jumped over
the fence
the fence was
too high
I guess they
------------------------------
Date: Sat, 1 Aug 2009 17:56:41 +0100
From: Ben Morrow <ben@morrow.me.uk>
Subject: Re: open file, delete 4 lines if they meet criteria
Message-Id: <9n8ek6-0ln2.ln1@osiris.mauzo.dyndns.org>
Quoth Faith Greenwood <fgnowfg@gmail.com>:
> OK, I know how to open a file and delete a specific line, but how can
> I delete a section of lines? for instance, I have:
You want the .. operator in scalar context.
<snip>
> The part I will be deleting will obviously be:
> but for some reason
> the fence was
> too high
> I guess they
> will have to try and try again
>
> and will alway start with "but for some reason" and end with "will
> have to try and try again" but the middle contents will change....how
> can I do this in perl?
perl -ne'print unless /but for/../will have to/'
or the equivalent while loop.
Ben
------------------------------
Date: Sat, 1 Aug 2009 10:51:15 -0700 (PDT)
From: Faith Greenwood <fgnowfg@gmail.com>
Subject: Re: open file, delete 4 lines if they meet criteria
Message-Id: <c2856868-da05-41e8-a612-db154b20a7fc@r38g2000yqn.googlegroups.com>
On Aug 1, 12:56=A0pm, Ben Morrow <b...@morrow.me.uk> wrote:
> Quoth Faith Greenwood <fgno...@gmail.com>:
>
> > OK, I know how to open a file and delete a specific line, but how can
> > I delete a section of lines? for instance, I have:
>
> You want the .. operator in scalar context.
>
> <snip>
>
> > The part I will be deleting will obviously be:
> > but for some reason
> > the fence was
> > too high
> > I guess they
> > will have to try and try again
>
> > and will alway start with "but for some reason" and end with "will
> > have to try and try again" but the middle contents will change....how
> > can I do this in perl?
>
> =A0 =A0 perl -ne'print unless /but for/../will have to/'
>
> or the equivalent while loop.
>
> Ben
dude, that is too easy! now that I see it I feel ashamed for even
asking. thx!
-Faith
------------------------------
Date: Sat, 01 Aug 2009 18:49:00 -0500
From: Tad J McClellan <tadmc@seesig.invalid>
Subject: Re: open file, delete 4 lines if they meet criteria
Message-Id: <slrnh79kma.8vl.tadmc@tadmc30.sbcglobal.net>
Faith Greenwood <fgnowfg@gmail.com> wrote:
> On Aug 1, 12:56 pm, Ben Morrow <b...@morrow.me.uk> wrote:
>> Quoth Faith Greenwood <fgno...@gmail.com>:
>>
>> > how can
^^^ ^^^
>> > I delete a section of lines?
^ ^^^^^
perldoc -q how
How can I pull out lines between two patterns that are
themselves on different lines?
perldoc -q can
How can I pull out lines between two patterns that are
themselves on different lines?
perldoc -q I
How can I pull out lines between two patterns that are
themselves on different lines?
perldoc -q lines
How can I pull out lines between two patterns that are
themselves on different lines?
>> perl -ne'print unless /but for/../will have to/'
> I feel ashamed for even
> asking.
That is appropriate.
You should always try searching the FAQ on a subject before doing
your part to ensure that the Question remains Frequently Asked...
--
Tad McClellan
email: perl -le "print scalar reverse qq/moc.noitatibaher\100cmdat/"
------------------------------
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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End of Perl-Users Digest V11 Issue 2533
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