in reply to Getting keys/values from a referenced hash

Hi there professsa,

In general, if you've got a reference to a hash - let's call it $ref - then you can work with it like this:

#construct a ref to an anonymous hash:
my $ref = {andy => 'likes cake'}; #note *curly* brackets
#another way
my %hash = (andy => 'likes beer too'); $ref = \%hash; #normal brackets
+ of course

#get at one specific value
print $ref->{andy};
#another way
print $$ref{andy};

#get all the keys
print keys %$ref;
#get all the values
print values %$ref;
#get key/value pairs
while (my ($key, $val) = each %$ref) {print "$key = $val"}

#copy the hash into a new hash
my %newhash = %$ref

#if you use the -> syntax with references to references, then you can 
+omit the ->s, after the first one, like this:
my $ref = { andy => {eats => 'cake'}};
print $ref->{andy}{eats}; #note only one -> necessary

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More info can be found here: perlref or here: References quick reference (or, newly, here if you speak German: (lang: de) Referenzen) or by typing 'perldoc perlref' at your command line.

hth,
andy.

update: and the book "Effective Perl Programming" has an excellent explanation, very full, with pictures.

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Re: dereferencing syntax Re: Getting keys/values from a referenced hash
by professa (Beadle) on Nov 28, 2001 at 15:40 UTC
    Hi Andy,

    I got a question concerning dereferencing hashes and use strict;.
    I have a reference to a hash, derived from a reference to a nested hash:

    my $hashref = $nested->{"key_to_hash"};

    When I dereference that $hashref via

    line 35:   my %realhash = %{$hashref};

    and when I "use strict" I get the following message:

    Can't use an undefined value as a HASH reference at
      /home/user/programming/perl/perlscript.pl line 35 (#1)
    (F) A value used as either a hard reference or a symbolic referenc
+e must
    be a defined value.  This helps to delurk some insidious errors.

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    Line 35 is the line where I dereference from $hashref.
    When I don't use strict everything works fine...I got no idea what means "defined value" here. All my refs and vars are defined, or am I wrong?

    Have a nice day and thanks for the tips yesterday!
    Micha

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      Hi again Misha, here's my $.2,

      Well, this line (which I think is equivalent) works ok: 

      perl -e 'my $nested = { key_to_hash => {data => "here"}}; print %{$nes
+ted->{key_to_has
h}};'

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      The error message simply means that $hashref is undefined, i.e. the scalar called 'hashref' contains no value. So this means that there is a problem with $nested->{key_to_hash} - maybe the key key_to_hash doesn't exist, or the value is undefined. Maybe $nested is pointing to the wrong hash.

      An easy way to see what's gone wrong is to use the module Data::Dumper - which is standard, so you probably have it. Do this:

      use Data::Dumper;
print Dumper($nested);

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      and you'll see the contents of the hash referred to by $nested.

      If you can't see what's up with it, post the code where you set up $nested.

      Have a nice day and thanks for the tips yesterday!
      That's ok, you too.

      andy.

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        Hi Andy!

        I found out now (using Dumper-mod) that my reference to the nested hash is undefined (but only when I use use strict;)
        When I don't use strict the reference ($results) holds all the data I want and is dumped correctly. Here's the way I get the nested hash from a file (the only possibility I have):

        my $filename = $ARGV[0];
open (_filehandle_, "<$filename") or die "Meeeeep! $filename does not 
+exist!\n";
my @file = <_filehandle_>;
close (_filehandle_);
my $line = join "", @file;
my $results = eval ($line);
print '$results: '.Dumper($results);
my $colpattern = $results->{"col_pattern"};

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        That's pretty straight and ineffective code, but I better first try how things work and squeeze it up later then. ;-)
        The fact that $results is undef tells me sth. goes wrong with eval().
        Can it be that the use strict-statement affects or somehow impairs the evaluation of the file's contents (which is in $line now)?

        Greetz, Micha

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        (whoops! I think I started a new thread accidentally by changing the subject line...)

        Hi Andy!

        I found out now (using Dumper-mod) that my reference to the nested hash is undefined (but only when I use use strict;)
        When I don't use strict the reference ($results) holds all the data I want and is dumped correctly. Here's the way I get the nested hash from a file (the only possibility I have):

        my $filename = $ARGV[0];
open (_filehandle_, "<$filename") or die "Meeeeep! $filename does not 
+exist!\n";
my @file = <_filehandle_>;
close (_filehandle_);
my $line = join "", @file;
my $results = eval ($line);
print '$results: '.Dumper($results);
my $colpattern = $results->{"col_pattern"};

        [download]

        That's pretty straight and ineffective code, but I better first try how things work and squeeze it up later then. ;-)
        As output from ::Dumper I get: $results: $VAR1 = undef;
        The fact that $results is undef tells me sth. goes wrong with eval().
        Can it be that the use strict-statement affects or somehow impairs the evaluation of the file's contents (which is in $line now)?

        Greetz, Micha

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