Hashes

Introduction

A hash is Perl's built-in way of storing data as key-value pairs, letting you look up a value instantly by a meaningful name instead of a numeric position. If arrays are for ordered lists, hashes are for labeled data — think of a dictionary, a phone book, or a configuration file.

What is a Hash?

A hash stores an unordered collection of key-value pairs. Every key is unique within a given hash, and each key maps to exactly one scalar value. Hash variable names use the % sigil.

use strict;
use warnings;

my %capitals = (
    "France"  => "Paris",
    "Japan"   => "Tokyo",
    "Kenya"   => "Nairobi",
);

print "Capital of Japan: $capitals{Japan}\n";

The => symbol, called the "fat comma," works exactly like a regular comma but visually pairs a key with its value, and it also automatically quotes a bareword on its left side, which is why "France" could just as easily be written without quotes as France =>.

Declaring a Hash

You can declare a hash empty and fill it in gradually, or initialize it all at once as shown above. Both approaches are common depending on whether your data is known upfront or built up as your script runs.

use strict;
use warnings;

my %inventory;   # empty hash

$inventory{"apples"}  = 50;
$inventory{"bananas"} = 30;
$inventory{"cherries"} = 120;

print "Apples in stock: $inventory{apples}\n";

Notice the sigil switch again, just like with arrays: the whole hash is %inventory, but a single value is accessed with $inventory{key}, because one value is a scalar.

Accessing and Updating Values

Reading and writing a hash value use the exact same $hash{key} syntax — Perl figures out from context whether you are reading or assigning.

use strict;
use warnings;

my %prices = (
    coffee => 3.50,
    tea    => 2.75,
);

print "Coffee costs \$$prices{coffee}\n";

$prices{coffee} = 4.00;   # update an existing value
$prices{juice}  = 3.25;   # add a brand new key

print "Updated coffee price: \$$prices{coffee}\n";
print "New juice price: \$$prices{juice}\n";

Checking for Keys and Deleting Entries

Before accessing a key that might not exist, it is good practice to check for it with the exists function, which avoids autovivification (Perl accidentally creating an empty entry just because you referenced it). The delete function removes a key-value pair entirely.

use strict;
use warnings;

my %prices = (
    coffee => 4.00,
    tea    => 2.75,
);

if (exists $prices{tea}) {
    print "Tea is on the menu at \$$prices{tea}\n";
}

if (!exists $prices{soda}) {
    print "Soda is not on the menu.\n";
}

delete $prices{tea};

if (!exists $prices{tea}) {
    print "Tea has been removed from the menu.\n";
}

keys and values

The keys function returns a list of all keys in a hash, and values returns a list of all corresponding values. Because a hash is inherently unordered, the order these come back in is not guaranteed to match your original insertion order.

use strict;
use warnings;

my %capitals = (
    France => "Paris",
    Japan  => "Tokyo",
    Kenya  => "Nairobi",
);

my @countries = sort keys %capitals;
print "Countries: @countries\n";

my @cities = sort values %capitals;
print "Cities: @cities\n";

Wrapping keys %capitals in sort gives predictable, alphabetical output — a very common pattern, since hash order alone cannot be relied on.

Looping Over a Hash

The most common way to process every key-value pair is to loop over the sorted keys and look up each corresponding value inside the loop.

use strict;
use warnings;

my %capitals = (
    France => "Paris",
    Japan  => "Tokyo",
    Kenya  => "Nairobi",
);

foreach my $country (sort keys %capitals) {
    print "$country -> $capitals{$country}\n";
}

Common Mistakes

Best Practices

Frequently Asked Questions

Key Takeaways

Summary

You now know all three of Perl's core data types: scalars, arrays, and hashes. Next, we will dig into one of Perl's most distinctive concepts — scalar versus list context — and revisit lexical scoping with my in more depth.