How to take a hash from Perl array?-CodePudding

This is the code:

my @items = (); # this is the array
my %x = {}; # the hash I'm going to put into the array
$x{'aa'} = 'bb'; # fill it up with one key-value pair
push(@items, $x); # add it to the array
my %i = @items[0]; # taking it back
print $i{'aa'}; # taking the value by the key

I expect it to print bb, but it doesn't print anything. What am I doing wrong?

CodePudding user response：

What am I doing wrong?

Well, this doesn't do what you think:

my %x = {};

You can easily check:

use Data::Dumper;
print Dumper \%x;

The output:

Reference found where even-sized list expected at ./1.pl line 5.
$VAR1 = {
          'HASH(0x556a69fe8220)' => undef
        };

The first line comes from use warnings;. If you don't use it, start now.

Why is the contents of the hash so strange? The curly braces create a hash reference. As keys must be strings, the reference is stringified to the HASH(0xADDR). You don't want a reference, you want a list of keys and values.

my %x = ();

In fact, each hash and array starts up empty, so you don't have to use anything:

my @items;
my %x;

Or, you can populate it right ahead instead of doing it separately on the next line:

my %x = (aa => 'bb');

Similarly, this doesn't work:

push @items, $x;

It pushes value of the scalar variable $x into @items, but there's no such variable. It seems you aren't using use strict;. Start now, it will save you from similar errors.

You want to push the hash to the array, but array values are scalars - so you need to take a reference of the hash instead.

push @items, \%x;

No, you want to retrieve the hash from the array. Again, this doesn't work:

my %i = @items[0];

You are extracting a single value. Therefore, you shouldn't use @. Moreover, we now have a reference in the array, to populate a hash, we need to dereference it first:

my %i = %{ $items[0] };

And voilà, it works!

#!/usr/bin/perl
use warnings;
use strict;

my @items;               # this is the array
my %x;                   # the hash I'm going to put into the array
$x{aa} = 'bb';           # fill it up with one key-value pair
push @items, \%x;        # add it to the array
my %i = %{ $items[0] };  # taking it back
print $i{aa};            # taking the value by the key