在Perl中的哈希中迭代哈希数组

Question

I have an Array of Hashes in a Hash that looks like this:

$VAR1 = {
          'file' => [
                      {
                        'pathname' => './out.log',
                        'size' => '51',
                        'name' => 'out.log',
                        'time' => '1345799296'
                      },
                      {
                        'pathname' => './test.pl',
                        'size' => '2431',
                        'name' => 'test.pl',
                        'time' => '1346080709'
                      },
                      {
                        'pathname' => './foo/bat.txt',
                        'size' => '24',
                        'name' => 'bat.txt',
                        'time' => '1345708287'
                      },
                      {
                        'pathname' => './foo/out.log',
                        'size' => '75',
                        'name' => 'out.log',
                        'time' => '1346063384'
                      }
                    ]
        };

How can I iterate through these "file entries" in a loop and access its values? Is it easier to copy my @array = @{ $filelist{file} }; so i only have an array of hashes?

Answer 1

No need to copy:

foreach my $file (@{ $filelist{file} }) {
  print "path: $file->{pathname}; size: $file->{size}; ...\n";
}

Answer 2

There are no arrays of hashes in Perl, only arrays of scalars. It only happens that there's a bunch of syntactic sugar in case those scalars are references to arrays or hashes.

In your example, $VAR1 holds a reference to a hash containing a reference to an array containing references to hashes. Yeah, that's quite a lot of nesting to deal with. Plus, the outer hash seems kinda useless, since it contains only one value. So yes, I think giving the inner array a meaningful name would definitely make things clearer. It's not actually a "copy": only the reference is copied, not the contents. All of the following are equivalent:

my @files = $VAR1 -> {file} # dereferencing with the -> operator
my @files = ${$VAR1}{file}  # derefencing with the sigil{ref} syntax
my @files = $$VAR1{file}    # same as above with syntactic sugar

Note that when using the sigil{ref} syntax, the sigil obeys the same rules as usual: %{$ref} (or %$ref ) is the hash referenced by $ref, but the element of %{$ref} for a given key is ${$ref}{key} (or $$ref{key} ). The braces can contain arbitrary code returning a reference, while the short version can only be used when a scalar variable already holds the reference.

Once your array of references to hashes is in a variable, iterating over it is as easy as:

for (@files) {
    my %file = %$_;
    # do stuff with %file
}

See: http://perldoc.perl.org/perlref.html