Perl Hashes

Hashes (also called associative arrays) are unordered collections of key-value pairs. They are prefixed with a % symbol and provide fast lookup by key.

Creating Hashes


# Simple hash creation
%person = (
    "name" => "John Doe",
    "age" => 30,
    "occupation" => "Programmer"
);

# Using the fat comma (=>) which auto-quotes left side
%colors = (
    red   => "#FF0000",
    green => "#00FF00",
    blue  => "#0000FF"
);

# From arrays (even number of elements)
@pairs = ("key1", "value1", "key2", "value2");
%hash = @pairs;

# Empty hash
%empty = ();

Accessing Hash Elements


# Accessing elements
print $person{"name"};  # "John Doe"
print $colors{"red"};   # "#FF0000"

# Checking existence
if (exists $person{"age"}) {
    print "Age exists\n";
}

# Deleting elements
delete $person{"occupation"};

Hash Operations


# Getting all keys
@keys = keys %person;  # ("name", "age", "occupation")

# Getting all values
@values = values %person;

# Iterating through hash
while (($key, $value) = each %person) {
    print "$key => $value\n";
}

# Hash size
$size = keys %person;  # Number of key-value pairs

# Clearing a hash
%person = ();  # Empty the hash
undef %person; # Alternative way

Hash Slices


# Accessing multiple values
@values = @person{"name", "age"};  # ("John Doe", 30)

# Setting multiple values
@person{"name", "age"} = ("Jane Doe", 32);

Special Hash Variables


# Environment variables
print $ENV{"PATH"};

# Command line switches
use Getopt::Long;
GetOptions(\%options, "help", "verbose");

# Signal handlers
$SIG{"INT"} = \&handle_interrupt;

# Special hashes
%INC;   # Hash of loaded modules
%ENV;   # Environment variables
%SIG;   # Signal handlers

Hash Functions


# Sorting keys
foreach $key (sort keys %hash) {
    print "$key: $hash{$key}\n";
}

# Sorting by value
foreach $key (sort { $hash{$a} <=> $hash{$b} } keys %hash) {
    print "$key: $hash{$key}\n";
}

# Merging hashes
%combined = (%hash1, %hash2);  # Later values overwrite earlier

# Filtering
%filtered = map { $_ => $hash{$_} } grep { condition } keys %hash;

Multidimensional Hashes


# Hash of hashes
%employees = (
    "1001" => {
        name => "John Doe",
        age => 30,
        dept => "IT"
    },
    "1002" => {
        name => "Jane Smith",
        age => 28,
        dept => "HR"
    }
);

# Accessing elements
print $employees{"1001"}{"name"};  # "John Doe"

# Adding new record
$employees{"1003"} = {
    name => "Bob Johnson",
    age => 35,
    dept => "Finance"
};

Best Practices

Use meaningful keys that describe the data
Check for key existence with exists before accessing
Consider using each for large hash iteration
Use hash slices for multiple value access
Free memory with undef when done with large hashes

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