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Non Existing Hash Key and Perl Function

Using Perl Hashes Part 5

Perl Course

Foreword: In this part of the series I explain the problem of passing a non-existing hash key to a Perl function and how to solve it.

By: Chrysanthus Date Published: 3 Nov 2015

Introduction

This is part 5 of my series, Using Perl Hashes. In this part of the series I explain the problem of passing a non-existing hash key to a Perl function and how to solve it. You should have read the previous parts of the series before coming here, as this is the continuation.

Passing a Non-Existing Hash Key to a Perl Function
Read and try the following code:

use strict;

    my %ha;

    sub fn
        {
            $_[0] = 'aa';
        }
  
    fn($ha{one});

    print $ha{one};

There should have been no output printed, but the output is,

    aa

This is wrong. The hash had an empty list. However, as the assumed key was passed to the function, and the assumed key as $_[0] being assigned a value, the hash developed the key/value pair. It does not have to be like that. That is a weakness. See solution below.

Passing a Non-Existing Hash Slice
If you pass a non-existing hash slice, even if there is no assignment in the function body, the hash develops the keys. Try the following code:

use strict;

    my %ha;

    sub fn
        {
            
        }
  
    fn(@ha{'one', 'two'});

    print "Hash now has a key as one\n" if exists($ha{'one'});
    print "Hash now has a key as two\n" if exists($ha{'two'});

The output is:

    Hash now has a key as one
    Hash now has a key as two

This output is wrong. It means the hash has developed keys when it was not supposed to; just because a slice with the assume keys has been passed to a function. See solution below.

The Hash::Util Module
This module that comes with Perl, can be used to stop addition of more keys into a hash. It can be used to stop more keys, but allow a list of possible names (keys) that can be added. You bring in the module to the program as follows:

  use Hash::Util qw(
                     hash_seed all_keys
                     lock_keys unlock_keys
                     lock_value unlock_value
                     lock_hash unlock_hash
                     lock_keys_plus hash_locked
                     hidden_keys legal_keys
                   );

You can then restrict the keys to the current keyset for a particular hash as follows:

    lock_keys(%hash);

The hash may not be empty; you can restrict it to possible keys of a keyset as follows:

  lock_keys(%hash, @keyset);

If there are already some keys, you can restrict it to possible additional keys as follows:

    lock_keys_plus(%hash, @additional_keys);

Try the following code and note that the assumed hash key could not be added to the hash, resulting from the function call (and assignment).

use strict;

  use Hash::Util qw(
                     hash_seed all_keys
                     lock_keys unlock_keys
                     lock_value unlock_value
                     lock_hash unlock_hash
                     lock_keys_plus hash_locked
                     hidden_keys legal_keys
                   );

    my %ha;

    lock_keys(%ha);

    sub fn
        {
            $_[0] = 'aa';
        }
  
    fn($ha{one});

    print $ha{one};

I tried the code and I had the following output (error message):

    Attempt to access disallowed key 'one' in a restricted hash at C:\temp.pl line 19.

This means that the key was not created as hoped.

Try the following code for a slice and function, and note that there is no creation of keys:

use strict;

  use Hash::Util qw(
                     hash_seed all_keys
                     lock_keys unlock_keys
                     lock_value unlock_value
                     lock_hash unlock_hash
                     lock_keys_plus hash_locked
                     hidden_keys legal_keys
                   );

    my %ha;

    lock_keys(%ha);

    sub fn
        {
            
        }
  
    fn(@ha{'one', 'two'});

    print "Hash now has a key as one\n" if exists($ha{'one'});
    print "Hash now has a key as two\n" if exists($ha{'two'});

I tried the code and I had the following output (error message):

    Attempt to access disallowed key 'one' in a restricted hash at C:\temp.pl line 22.

This means that non of the assumed keys from the hash slice was created.

That is it, for this part of the series.

Chrys

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