SPUG: MORE INFO: Array Naming Question
North, Walter
wnorth at state.mt.us
Wed Feb 4 11:46:38 CST 2004
Dan:
>>>>>>>Blinding Flash of Light<<<<<<<<<
Sometimes the obvious is just too obvious.............
Its the key stupid, not the hash name...........
Thank you.
Also Chris Wilkes for the thought on creating my own
perl module. That is definately one I had not thought of.
Perhaps as my skills increase........
thanks to all
> -----Original Message-----
> From: Dan Ebert [mailto:mathin at mathin.com]
> Sent: Wednesday, February 04, 2004 10:31 AM
> To: North, Walter
> Cc: Spug (E-mail)
> Subject: Re: SPUG: MORE INFO: Array Naming Question
>
>
>
> Could you make the key like this:
>
> my $base = 'storage_array_';
>
> my $number = 1; # this can be the part that can be dynamic
>
> my $key = $base . $number;
>
> $hash{$key} = %date_based_hash;
>
> On Wed, 2004-02-04 at 09:19, North, Walter wrote:
> > > I curious to know what problem is trying to be solved by
> > > using a string
> > > as the name of the variable.
> >
> > Dan:
> >
> > I collect some data from a storage array and read it into a hash
> > with the date as the key from which I then produce a graph
> for perusal
> > by management.
> >
> > More storage arrays are on the way and I'd like to combine the data
> > however I haven't been able to come up with how to create a hash
> > with the storage array name as a hash that contains another hash
> > of the data keyed by date.
> >
> > So I figured I could turn out multiple hashes with the date as
> > a key.
> >
> > However I would prefer not to have to modify the script to
> add another
> > hash each time we get another storage device. Hence I would want
> > to increment the name for example:
> >
> > ST_ARRAY_1 and the next would be ST_ARRAY_2 etc.
> >
> > A hash of hashes seems to be the superior method, but while I have
> > done hashes of arrays I cannot seem to get a satisfactory
> hash of hashes.
> >
> > Hence my question.
> >
> > As Christopher and Jeremy pointed out using a variable name
> for a variable
> > name is asking for trouble.
> >
> >
> > > -----Original Message-----
> > > From: Dan Ebert [mailto:mathin at mathin.com]
> > > Sent: Wednesday, February 04, 2004 9:40 AM
> > > Cc: spug
> > > Subject: RE: SPUG: Array Naming Question
> > >
> > >
> > > I curious to know what problem is trying to be solved by
> > > using a string
> > > as the name of the variable.
> > >
> > > Maybe you could use a hash where the string is the key and
> > > the value is
> > > the array (or hash, or whatever)?
> > >
> > > i.e.
> > >
> > > my %names = (array1 => [1,2,3],
> > > array2 => [4,5,6]);
> > >
> > > my $use_this = 'array1';
> > >
> > > foreach( @{ $names{$use_this} } ) { print; }
> > >
> > >
> > >
> > > Another possibility could be to use references.
> > >
> > > my @array1 = (1,2,3);
> > > my @array2 = (3,4,5);
> > >
> > > my $arrayref = \@array1;
> > >
> > > my $use_this = 'array1';
> > >
> > > if($use_this eq 'array2') { $arrayref = \@array2; }
> > >
> > > foreach(@$arrayref) { print; }
> > >
> > >
> > >
> > > On Wed, 2004-02-04 at 08:13, Cantrall, Christopher W wrote:
> > > > Also, MJD has an article on this subject.
> > > http://perl.plover.com/varvarname.html
> > > >
> > > >
> > > > BeginQuote
> > > >
> > > > The real root of the problem code is: It's fragile. You're
> > > mingling unlike things when you do this. And if two of those
> > > unlike things happen to have the same name, they'll collide
> > > and you'll get the wrong answer. So you end up having a whole
> > > long list of names which you have to be careful not to reuse,
> > > and if you screw up, you get a very bizarre error. This is
> > > precisely the problem that namespaces were invented to solve,
> > > and that's just what a hash is: A portable namespace.
> > > >
> > > > ...
> > > >
> > > > The real problem is that if your string contains something
> > > unexpected, it will sabotage a totally unrelated part of the
> > > program, and then you will have one hell of a time figuring
> > > out the bug.
> > > >
> > > > EndQuote
> > > >
> > > >
> > > > MJD has 3 articles on this. Quite interesting.
> > > >
> > > > HTH
> > > >
> > > > ____________________________________________
> > > > Chris Cantrall, Structural Engineer, 777
> > > > Christopher.W.Cantrall at Boeing.com
> > > > chris at cantrall.org
> > > > http://perlmonks.org/index.pl?node=Louis_Wu
> > > > http://spugwiki.perlocity.org/index.cgi?LouisWu
> > > >
> > > > > -----Original Message-----
> > > > > From: Jeremy G Kahn [mailto:kahn at cpan.org]
> > > > > Sent: Wednesday, February 04, 2004 7:53 AM
> > > > > To: North, Walter
> > > > > Cc: spug
> > > > > Subject: Re: SPUG: Array Naming Question
> > > > >
> > > > >
> > > > > This is a good question, and a FAQ if you know where to
> > > look. The FAQ
> > > > > has an explanation for how you might want to go about
> > > doing this, and
> > > > > why it's probably a bad idea, and some suggestions for
> > > alternatives:
> > > > >
> > > > > perldoc -q 'variable name':
> > > > >
> > > > > Found in /usr/share/perl/5.8.2/pod/perlfaq7.pod
> > > > > 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. That means that
> if $fred is
> > > > > a lexical variable created with my()
> in the above
> > > > > example, the code wouldn't work at all: you'd
> > > > > ...
> > > > >
> > > > > Hope that's useful.
> > > > >
> > > > >
> > > > > --jeremy
> > > > >
> > > > > North, Walter wrote:
> > > > >
> > > > > >Good Morning all,
> > > > > >
> > > > > >Maybe this is a dumb question, but here goes anyway:
> > > > > >
> > > > > >Does anyone know if it possible to include a variable in, or
> > > > > use a variable
> > > > > >as
> > > > > >the name of an array or hash, and if so how would one do it?
> > > > > >
> > > > > >thanks in advance.
> > > > > >
> > > > > >
> > > > > >-----------------------------------------------------
> > > > > >Walter North 406-444-2914
> > > > > >Operating Systems Programmer
> > > > > >wnorth (at) state (dot) mt (dot) us
> > > > > >-----------------------------------------------------
> > > > > >
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> > > > >
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