The Drivel is in the Details

dalgetty has asked for the wisdom of the Perl Monks concerning the following question:

O Holy Brethren,

My code works fine. It's just that I am using it in a manual, and I have to justify and explain each detail. This section of my package takes a hash-array-hash from XML::Simple and prints out the contents successfully

 
foreach my $node(keys (%$source)) {
      my $elem=$source->{$node};
      my $count=0;
      foreach my $attr (@$elem) {        
         $self->print.="\n\n$node $count\n\n";
         if ($count<1) {$self->nodename=$node;}
         foreach my $subnode(keys(%$attr)) {
                 $self->print.=$subnode."\t".$attr->{$subnode}."\n";
         }
         $count++;
      }
 }
[download]

Why is the -> necessary in $source->{$node}, and why the %$ and @$ symbols instead of just % and @?

In smaller, non-module scripts I use this kind of format:

foreach $label (keys %contact) {
print"$label:$contact{$label}\n";
[download]

Forgive my ignorance, and peace be with you

Dalgetty

Comment on The Drivel is in the Details Select or Download Code

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Re: The Drivel is in the Details by jZed (Prior) on May 29, 2007 at 22:24 UTC
$source is a reference to a hash. You find the keys by de-referencing it (%$source) then you dereference it with the arrow in $source->{node}. You could use the format you are used to if you do something like: `my %newSource = %$source; # dereference it all at once foreach my $node(keys (%newSource)) { # note no %$ #... $newSource{$node}; # already dereferenced.` [download] update I'm not recommending that you change the code, but showing a similar situation to explain the code. If you are thinking of changing the code, see the important caveat mentioned by Random_Walk.	[reply] [d/l]
Re^2: The Drivel is in the Details by Anonymous Monk on May 29, 2007 at 22:28 UTC
Thank you - I think I can explain it from there. Peace	[reply]
Re^2: The Drivel is in the Details by Random_Walk (Prior) on May 30, 2007 at 22:48 UTC
I think you should point out that when you do `my %newSource = %$source; # dereference it all at once` [download] you are not just dereferencing `$source` but making a new and independent copy in `%newSource`. This may have implications if the code evolves. Any change made to one array will not be reflected in the other, for instance an element removed from `%$source` is not removed from `%newSource` and the removal of `$source` will not imply removal of `%newSource` Cheers, R. Pereant, qui ante nos nostra dixerunt!	[reply] [d/l] [select]
Re: The Drivel is in the Details by FunkyMonk (Bishop) on May 29, 2007 at 22:40 UTC
XML::Simple returns a reference to a hash. `%$` and `@$` are used to get a hash and array respectively from a reference (dereferencing) `->` is shorthand for dereference-and-access of an array or hash `->` allows you to use $source->{$node} instead of %{$source}{$node} `->` is also used to call an object's method as you did in `$self->print`. $self is the object and print is the method	[reply] [d/l] [select]
Re: The Drivel is in the Details by blazar (Canon) on May 29, 2007 at 23:01 UTC
Why is the `->` necessary in `$source->{$node}` Because `$source` is a (hash) reference and it needs to be dereferenced. and why the `%$` and `@$` symbols instead of just `%` and `@`? Because the symbols are still simply `%` and `@`, but now you're using them to dereference something that is held in a scalar, which is prefixed by the `$` sigil: this is another method for dereferencing, and in its full blown form it should be: `%{ $hashref }` [download] But in this simple case the curlies can be omitted. In more complex contructs, they cannot.	[reply] [d/l] [select]
Re: The Drivel is in the Details by BenHopkins (Sexton) on May 31, 2007 at 04:10 UTC
%$ and @$ indicate that the variable is a hash-REF and an array-REF. Kinda like a C cast. Don't know about the -> in $source->{$node}, though.	[reply]
Re: The Drivel is in the Details by chexmix (Hermit) on May 31, 2007 at 18:45 UTC
re: "Why is the -> necessary in $source->{$node}" IIRC, in the "Alpaca book" the authors make the point that this arrow is optional "between subscripty kinds of things" but NOT in a case like this. perlreftut also states: "In between two subscripts, the arrow is optional." If you are working with an array reference, the perlreftut continues, "instead of `$a[1]->[2]`, (you) can write `$a[1][2]`."	[reply] [d/l] [select]