Unable to properly merge hashes

Question

I currently have the following

# $dog, $cat, $rat are all hash refs

                my %rethash = ('success' => 'Your Cool'); 

                my %ref ={ 'dog' => $dog,  'cat' => $cat,  'mouse' => $rat,
                                'chicken' => ''  };

                my $perlobj = ( \%ref,\%rethash );

When $perlobj is dumped this is the result

 $VAR1 = {
      'success' => 'Your Cool'
    };

However when warnings are enabled I get the following message

Useless use of reference constructor in void context at ..

I realize there is something terribly wrong with how %ref is assigned using {}, What is wrong with this code? I can't seem to get rid of this warning....

EDIT: Ok I think I figured out whats going on,

my $perlobj = ( \%ref,\%rethash );

This does not merge but results in $perlobj becoming a reference to %rethash, this is obvious after reading your responses.

You're use `{}` when creating `%ref` when you should be using `()` — RobEarl, Jun 02 '15 at 15:38
possible duplicate of [How can I combine hashes in Perl?](http://stackoverflow.com/questions/350018/how-can-i-combine-hashes-in-perl) — RobEarl, Jun 02 '15 at 15:46
You are wildly mixing `()` parens and `{}` as well as hashes and hash-references. And you have found a rare instance of `,` as a sequence operator instead of a list constructor. — Patrick J. S., Jun 02 '15 at 16:02
@PatrickJ.S. could you elaborate on the sequence operator? I cannot find that in perldoc. — simbabque, Jun 02 '15 at 16:13
@simbabque: In the docs it's just called ["comma" operator](http://perldoc.perl.org/perlop.html#Comma-Operator). I was reffering to [this usage of _sequence_](https://en.wikipedia.org/wiki/Comma_operator#Syntax). Admittedly the expresseion "as a sequence operator" is a bit sloppy and could have better been said with “used the `,`-operator for sequencing”, — Patrick J. S., Jun 02 '15 at 18:56

score 5 · Accepted Answer · answered Jun 02 '15 at 16:11

What RobEarl is saying is correct. I'll give an explanation of that and add some more stuff.

Your variable name %ref and the fact that you are using {} kinda implies you want a reference here.

Let's take a look what value we will have in %ref. Consider this example.

use strict; use warnings;
use Data::Printer;
my %foo = { key => 'value' };
p %foo;

This will throw a warning Reference found where even-sized list expected on my Perl 5.20.2. The output will be:

{
    HASH(0x7e33c0)   undef
}

It's a hash with a hashref as the key and undef as a value. HASH(0x07e33c0) is what you get when you look at a hash reference without dereferencing it. (The {} are there because Data::Printer converts the hash to a hashref).

Back to your code, the correct sigil for a reference is $. It does not matter what kind of reference it is. The reference is always a scalar (a pointer to the place in memory where the hash/array/something) is stored.

my $ref = { 
  dog     => $dog,
  cat     => $cat,
  mouse   => $rat,
  chicken => '', # maybe this should be undef?
};

Now you've got a hashref with the values of $dog, $cat, $rat and an empty string.

Now you're assigning a variable named $perlobj, which implies it's an object. Instead you are assigning a scalar variable (the $ makes it a scalar) with a list. If you do that, Perl will only assign the right-most value to the variable.

my $foo = (1, 2, 3); # $foo will be 3 and there's a warning

You are assigning a list of two references. The first one is disregarded and only \$rethash gets assigned. That works because conveniently, $perlobj is a scalar, and references are also scalars. So now $perlobj is a reference of %rethash. That's why your Data::Dumper output looks like %rethash.

I'm not sure what you want to do, so I cannot really help you with that. I suggest you read up on some stuff.

perlreftut is useful to learn how references work
If you want to do Object Oriented Programming, check out Moose
It might also be useful to just go get a book to learn a bit more about basic Perl. Learning Perl by Randal L. Schwartz and Beginning Perl by Curtis Poe are both very good for that

Thanks for breaking it down I really appreciate it, I was going through some old code and added warnings to a module and found this error. — ingueni, Jun 02 '15 at 17:32

score 2 · Answer 2 · answered Jun 02 '15 at 16:25

2

You are taking a list of hash references and assigning them to a scalar

my $perlobj = ( \%ref, \%rethash );  # same as  $perlobj = \%rethash

Instead you want to take a reference to a merger of hashes

my $perlobj = { %ref, %rethash };

answered Jun 02 '15 at 16:25

mob

117,087
18
149
283

Unable to properly merge hashes

2 Answers2