Hashes

Sometimes you'll still see people call them "associative arrays",but don't think of them as arrays.

Create hashes as lists of key/value pairs

Create a hash with a list of key/value pairs.

The is a "fat comma", and is identical to a comma, plus it quotes the previous word:

  1.     my %stooges = (
  2.         Moe => 'Howard',
  3.         Larry => 'Fine',
  4.         Curly => 'Howard',
  5.         Iggy => 'Pop',
  6.     );

A hash in list context becomes a list of key/value pairs.

  1.     my @hash_as_an_array = %stooges;
  2.     # Contains ( 'Curly', 'Haward', 'Larry', 'Fine', etc... )

Just as the order of keys and values from a hash is effectively random, the order of the key/value pairs in a flattened hash is effectively random.

Access individual hash entries with squiggle braces

Get hash values with squiggly braces instead of square braces.

  1.     print $stooges{'Iggy'};
  2.     # Prints "Pop"

Set values that way, too.

Overwrite existing values:

  1.     $stooges{'Iggy'} = 'Ignatowski';
  1.     delete $stooges{'Curly'};

Note that delete does not delete a file. unlink does that.

  1.     unlink $stooges{'Moe'};
  2.     # Deletes a file called 'Howard';

Get the keys/values of a hash

Use the keys and values keywords:

The order of the keys and values are guaranteed to match up to each other.

If your hash key is a single word, you don't need quotes around it.

    Hashes can only contain scalars

    The values in a Perl hash may only be scalars. It is not possible to contain an array or a list in an array.

    1.     $hash{comedians} = @stooges;
    2.     # Assigns the length of @stooges to the value

    If you want to store an array in a hash, you'll need to use references.

    Hashes are unordered

    The order of and values %hash is effectively random. They will differ between runs of the program. It is not related to the order in which things were added.

    You cannot "sort a hash"

    The idea of "sorting a hash" does not exist in Perl, because hashes are unordered. You can sort the keys of a hash, or the values of a hash, as they are just lists.

    1.     my %new_hash = (%hash1, %hash2);

    The right-hand side of the equals is a long list of key/value pairs from both of the hashes. The list is then assigned to %new_hash. If any keys in %hash2 duplicate keys in %hash1, the key/value pair take priority because they're assigned later.

    When to use hash, when to use array

    If you have things in a line, ordered, in a sequence, use an array.

    • A list of files to read
    • A list of people in a queue

    If you have a bag of things, unordered, that you want to look up, use a hash.

    • An index of last names, looked up by first name
    • An index showing the size of files, looked up by name

    To add (patches welcome!)

    The difference between defined and exists

    Use defined to see whether a hash element has a value other than undef. It will return true if the hash element has any value other than undef, even 0 and "" (the empty string), which evaluate to false.

    Use exists to see whether a hash element has ever been initialized, even if it is not defined (i.e., it has the value 'undef').

    A hash element can be true only if it's defined. It can be defined only if it exists.

    However, a hash element can exist without being defined. This means it will not return true even though it exists.

    1.     if ( $h{'foo'} ) {
    2.         print 'true';
    3.     }
    4.     else {
    5.         print 'false';
    6.     }
    7.     # prints 'false'; since $h{'foo'} is not defined, it cannot be true

    Access hash slices

    Submit a PR to