Perl Hash

Initialize (clear, or empty) a hash

Assigning an empty list is the fastest method.
Solution

    my %hash = ();

Initialize (clear, or empty) a hash reference

People have asked how to initialize a hash reference (aka hash ref and href). This is the way to go:
Solution

    my $hash_ref = {};  # a reference to an empty hash, ref will return HASH

The great thing about this is that if before performing an actual assignment, you want to determine (using the ref operator) the type of thingy that a reference is pointing to, you can!... and you can expect it to be a HASH built-in type, because that is what the line above initializes it to be.
Note
If you treat the variable just as any scalar variable; and use the my declaration alone, or assign a value, ref will return false.

    my $hash_ref;
    my $hash_ref = 0;  # zero

Add a key/value pair to a hash

In the solutions below, quotes around the keys can be omitted when the keys are identifiers.
Hash:
Solution

    $hash{ 'key' } = 'value';    # hash

    $hash{ $key } = $value;      # hash, using variables

Hash reference:
Solution

    $href->{ 'key' } = 'value';  # hash ref

    $href->{ $key } = $value;    # hash ref, using variables

Add several key/value pairs to a hash

Solution
The following statements are equivalent, though the second one is more readable:

    %hash = ( 'key1', 'value1', 'key2', 'value2', 'key3', 'value3' );

    %hash = (
        key1 => 'value1',
        key2 => 'value2',
        key3 => 'value3',
    );

Copy a hash

Solution

    my %hash_copy = %hash;  # copy a hash

    my $href_copy = $href;  # copy a hash ref

Delete a single key/value pair

The solution differs for a hash and a hash reference, but both cases can use the delete function.
Solution
Hash:

    delete $hash{$key};

Hash reference:

    delete $hash_ref->{$key};

Perform an action on each key/value pair in a hash

The actions below print the key/value pairs.
Solution
Use each within a while loop. Note that each iterates over entries in an apparently random order, but that order is guaranteed to be the same for the functions keys and values.

    while ( my ($key, $value) = each(%hash) ) {
        print "$key => $value\n";
    }

A hash reference would be only slightly different:

    while ( my ($key, $value) = each(%$hash_ref) ) {
        print "$key => $value\n";
    }

Solution
Use keys with a for loop.

    for my $key ( keys %hash ) {
        my $value = $hash{$key};
        print "$key => $value\n";
    }

Example

    my $file = $ARGV[0] || "-";
     
    my %from = ();
     
    open FILE, "< $file" or die "Can't open $file : $!";
     
    while( <FILE> ) {
        if (/^From: (.*)/) { $from{$1}++ }  # count recurrences of sender
    }
     
    close FILE;
     
    for my $sender ( sort keys %from ) {
        print "$sender: $from{$sender}\n";
    }

Get the size of a hash

Solution

    print "size of hash:  " . keys( %hash ) . ".\n";

Solution

    my $i = 0;
     
    $i += scalar keys %$hash_ref;  # method 1: explicit scalar context
    $i += keys %$hash_ref;         # method 2: implicit scalar context

Use hash references

Solution

    sub foo
    {
        my $hash_ref;
     
        $hash_ref->{ 'key1' } = 'value1';
        $hash_ref->{ 'key2' } = 'value2';
        $hash_ref->{ 'key3' } = 'value3';
     
        return $hash_ref;
    }
     
    my $hash_ref = foo();
     
    print "the keys... ", sort keys %$hash_ref, "...\n";

Create a hash of hashes; via references

The following two solutions are equivalent, except for the way the look. In my opinion the second approach is clearer.
Solution

    $requiredPatches_href->{ $patch }->{ os }    = $os;
    $requiredPatches_href->{ $patch }->{ arch }  = $arch;
    $requiredPatches_href->{ $patch }->{ info }  = $info;

Solution

    $requiredPatches_href->{ $patch } = {
                                          os    => $os,
                                          arch  => $arch,
                                          info  => $info,
                                        };

Function to build a hash of hashes; return a reference

Solution

    sub foo
    {
        my ( $login, $p, $uid, $gid, $gecos, $dir, $s );
     
        my %HoH = ();
     
        my $file = '/etc/passwd';
        open( PASSWD, "< $file" ) or die "Can't open $file : $!";
     
        while( <PASSWD> ) {
            ( $login, $p, $uid, $gid, $gecos, $dir, $s ) = split( ':' );
     
            $HoH{ $login }{ 'uid' } = $uid;
            $HoH{ $login }{ 'gid' } = $gid;
            $HoH{ $login }{ 'dir' } = $dir;
        }
     
        close PASSWD;
     
        return \%HoH;
    }

Access and print a reference to a hash of hashes

Solution

    my $rHoH = foo();
     
    my( $uid, $gid, $dir );
     
    for my $login ( keys %$rHoH ) {
     
        $uid =       $rHoH->{ $login }->{ 'uid' };   # method 1  most readable
        $gid =    ${ $rHoH->{ $login } }{ 'gid' };   # method 2
        $dir = ${ ${ $rHoH }{ $login } }{ 'dir' };   # method 3 least readable
     
        print "uid: $uid, gid: $gid, dir, $dir.\n";
    }

Solution

    my $rHoH = foo();
     
    for my $k1 ( sort keys %$rHoH ) {
        print "k1: $k1\n";
        for my $k2 ( keys %{$rHoH->{ $k1 }} ) {
            print "k2: $k2 $rHoH->{ $k1 }{ $k2 }\n";
        }
    }

Function to build a hash of hashes of hashes; return a reference

Solution

    sub foo
    {
        my %HoHoH = ();
     
        while( ... ) {
     
            if( /LOCATION:/ ) {
     
                ...
     
            } elsif( /MODULE:/ ) {
     
                $HoHoH{ $loc }{ $module_type }{ MODULE_NAME } = $module_name;
     
            } elsif( $ARGS_ALLOWED ) {
     
                $HoHoH{ $loc }{ $module_type }{ $arg_name } = $arg_value;
     
            }
     
        }
     
        return \%HoHoH;
    }

Access and print a reference to a hash of hashes of hashes

Solution

    my $rHoHoH = foo();
     
    for my $k1 ( sort keys %$rHoHoH ) {
        print "$k1\n";
     
        for my $k2 ( sort keys %{$rHoHoH->{ $k1 }} ) {
            print "\t$k2\n";
     
            for my $k3 ( sort keys %{$rHoHoH->{ $k1 }->{ $k2 }} ) {
                print "\t\t$k3 => $rHoHoH->{ $k1 }->{ $k2 }->{ $k3 }\n";
            }
        }
    }

Print the keys and values of a hash, given a hash reference

Solution

    while( my ($k, $v) = each %$hash_ref ) {
        print "key: $k, value: $v.\n";
    }

Determine whether a hash value exists, is defined, or is true

Solution

    print "Value EXISTS, but may be undefined.\n" if exists  $hash{ $key };
    print "Value is DEFINED, but may be false.\n" if defined $hash{ $key };
    print "Value is TRUE at hash key $key.\n"     if         $hash{ $key };

Example
Let's say we execute an sql query where some of the resulting values may be NULL. Before attempting to use any of the values we should first check whether they are defined, as in the following code. Note that the subroutine sql_fetch_hashref() takes care of connecting to the database, preparing the statement, executing it, and returning the resulting row as a hash reference using DBI's fetchrow_hashref() method.

    my $answers = 'a,b,c,d,e';
     
    my $sql = "select max_time, $answers from questions " .
              'where question_number=?';
    my $hash_ref = sql_fetch_hashref( $sql, $q );
     
    my @answers = split ',', $answers;
     
    my $max_time = $hash_ref->{max_time} || '60';
     
    my $hash_ref_ans;
    for my $letter ( @answers ) {
        $hash_ref_ans->{ $letter } = $hash_ref->{ $letter }
            if defined $hash_ref->{ $letter };
    }

The for loop made a new hash of only defined key/value pairs.