How to add a scalar value (string) to an existing hash in Perl

Question

I just want to know what the process is behind merging a value into a hash. I have a hash which has 5 to 6 keys depending on if the error outputs runtime values. The method that takes in the arguments also take an error message string in first, also. I want it to be able to add this error message string into the hash, to make one big hash basically.

This is how the method would be called:

ASC::Builder::Error->new("Simple error message here", code => "UNABLE_TO_PING_SWITCH_ERROR", switch_ip => $ip3, timeout => $t1);

The last two values assign runtime parameters/values to keys inside the context key in the error hash.

Here is a look at the error hash:

 use constant ERROR_CODE => {
      UNABLE_TO_PING_SWITCH_ERROR => {
          category => 'Connection Error',
          template => 'Could not ping switch %s in %s seconds.',
          context => [qw(switch_ip timeout)],
          tt => {template => 'disabled'},
          fatal => 1,
          wiki_page => 'www.error-solution.com/ERROR_CODE_A',
      }
 };

Here is my method to manipulate the error hash and construct the message

sub _create_error_hash { 
    my $error_string = shift;                                                                                                                                                              if(defined($params{code}) {                    
    my $first_param = delete $params{code};                                                                                                                                                            
        foreach my $key (@{$first_param->{context}}) {                                                                                                                                                     
            $first_param->{$key} = $key;                                                                                                                                                                   
        }                                                                                                                                                                                                  
        my @template_args = map { $first_param->{$_}} @{$first_param->{context} };                                                                                                                         
        $first_param->{message} = sprintf($first_param->{template}, @template_args);                                                                  }                                                     
        return bless $first_param;                                                                                                                                                                         
    }

sub _merge_hashes {
    my ($message = {message => $messsage}, $first_param = {first_param => $first_param}) = @ _;
    #merge these two hashes and bless into $class (don't know how to merge hashes)
    my %merged_hash = ($message, $first_param);
    return bless $merged_hash, $class;
}

The output of _create_hash should be the input for _merge_hashes Not sure if I have handled that properly. These methods will be use inside the new method (which is a mess right now) hence why it's not included.

That's just an attempt , of an example I seen on perlmonks, Here is the link: http://www.perlmonks.org/?node_id=14263

Just a note -- remember that you want the constructor to return a proper object, so the `new` method should return a `bless`-ed ref so that the calling code has a proper thing to work with. Normally it is `new` that will do `return bless $self, $class`. Inside of that method, so inside the class, you can of course call other subs to organize work. — zdim, Jun 29 '16 at 17:41

score 1 · Accepted Answer · answered Jun 29 '16 at 15:06

I'm going to start with the simple explanation of how to merge a hash in perl, it's pretty simple.

use strict;
use warnings;

use Data::Printer;

my (%a, %b, %c);

%a = (a => 1, b => 2);
%b = (a => 0, c => 3, d => 4);
%c = (%a, %b);

p(%c); # { a => 0, b => 2, c => 3, d => 4 }

You'll note with the a keys that if there are duplicates whatever value appears in the second set will be the one that 'wins'.

I honestly have no idea what that second function is doing because it references variables that don't exist on practically every line. (The create one also does this but only on a couple lines).

From your description I think you only want a single key added so you don't really need to do that though, you should be able to just add the key to the original object: $error->{messsage} = $message

But assuming you did want to pass two hash references in and merge them, it would look something like this:

sub merge {
    my ($first, $second) = @_;
    my %merged = (%$first, %$second);
    return bless \%merged, $class;
}

yeah I only want a single key, my thinking of using references was that the hashes are already created in the new() method which consumes both of these. This has given me a better understanding, I'm just not sure when passing the output of one into the other, should I be blessing outputs `$first_param` is meant to be `"UNABLE_TO_PING SWITCH_ERROR"` , after code is removed. — Paul Russell, Jun 29 '16 at 15:21
All blessing does is associate a package name with a given reference for use in method lookup, so usually you would just have it on the top level object. — Tim Tom, Jun 29 '16 at 15:34
And if you were saying you want the error message to be the key of the value, you can do that `$error->{$message} = $hashRef` — Tim Tom, Jun 29 '16 at 15:35

How to add a scalar value (string) to an existing hash in Perl

Here is my method to manipulate the error hash and construct the message

1 Answers1