Perl: Adding more elements to array created from hash

Question

I have a perl script where we are converting Hash into array-

my %info = (
          'name'    => $1,
          'ip'      => $2,
          'parent'  => '',
          'isLocal' => 0,
          'clip'    => 0,
          );
my @siteinfos = ();
push( @siteinfos, \%info );

Now I am able to accesss the array by-

foreach my $site(@siteinfos) {
if ( $site->{'name'} eq  $sitename)  {
 .....
}

First I am not sure how this conversion actually works.Secondly,Now I want to add more elements to this array in (key,pair) format.How can I do so?

... and besides that one (see this answer), try this question. — cxw
– cxw, Commented Jan 17, 2017 at 14:52

Sinan Ünür · Accepted Answer · 2017-01-17 15:02:35Z

7

First, there is no conversion.

my @siteinfos = ();

defines an array @siteinfos and sets to the empty list. Of course, you could have achieved the same result by using just

my @siteinfos;

Next, the statement

push( @siteinfos, \%info );

pushes a reference to the hash %info to @siteinfos as the sole element of that array.

Next, the loop

foreach my $sinfo (@siteinfos) { # assuming original is a typo

ends up aliasing $sinfo to that sole hash reference in @siteinfos. In the body of the loop, you access an element of the original hash via the reference you pushed to @siteinfos.

Now, things would have been different if you had done:

push @siteinfos, %info;

That would have set @siteinfos to a list like the following (the order is non-deterministic):

$VAR1 = [
          'ip',
          undef,
          'clip',
          0,
          'isLocal',
          0,
          'name',
          undef,
          'parent',
          ''
        ];

The undef are there because the match variables were empty when I ran the script. Now, if you want to push another key-value pair to this array, that's trivial:

 push @siteinfos, $key, $value;

But of course, by assigning the hash to an array, you are losing the ability to simply look up values by key. In addition, you can keep pushing duplicate keys to an array, but if you assign it back to another hash:

my %otherinfo = @siteinfos;

then only the last version of an even-indexed element (key) and its corresponding odd-indexed element (value) will survive.

edited Jan 17, 2017 at 15:02

answered Jan 17, 2017 at 14:56

Sinan Ünür

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3 Comments

simbabque Over a year ago

We not only have the same kind of answer, we even used the same way to format it. :D

Sinan Ünür Over a year ago

@simbabque If you want to combine the second part of my answer regarding pushing more key-value pairs to the array, I'll delete mine.

simbabque Over a year ago

Nonono, you only need 5k more rep for your t-shirt. ;) We can all have a Stack Overflow t-shirt party at YAPC::EU in Amsterdam this summer.

simbabque · Accepted Answer · 2017-01-17 14:57:06Z

5

You are not converting anything. You are making a new array named @siteinfos.

my @siteinfos = ();

Then you add one entry, which is a reference to the hash %info.

push( @siteinfos, \%info );

The array now holds a reference to %info. That's $siteinfo[0].

In your loop you iterate all the elements in @siteinfos. The hashref now goes into $sinfo.

If you want to add more keys and values to that hashref, just put them in.

foreach my $sinfo (@siteinfos) {
    if ( $sinfo->{'name'} eq  $sitename)  {
        $sinfo->{foo} = "bar"; # as simple as this
    }
}

answered Jan 17, 2017 at 14:57

simbabque

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