Perl Data Types

Overview

Perl is a weakly typed language but has three main data types:

1. Scalars: Store single values
2. Arrays: Store ordered lists of values
3. Hashes: Store key-value pairs

Each data type has its unique sigil:

• Scalars: $
• Arrays: @
• Hashes: %

Scalars

Scalar Value Types

Scalars can store the following types of values:

use strict;
use warnings;

# Numbers
my $integer = 42;          # Integer
my $float = 3.14;          # Floating point
my $scientific = 1.23e-4;  # Scientific notation

# Strings
my $string1 = "Hello";     # Double-quoted string
my $string2 = 'World';     # Single-quoted string
my $empty = "";            # Empty string

# Special values
my $undef = undef;         # Undefined value
my $true = 1;              # True value
my $false = 0;             # False value

# References
my $array_ref = [1, 2, 3];       # Array reference
my $hash_ref = {a => 1, b => 2}; # Hash reference

Double vs Single Quotes

my $name = "Alice";

# Double quotes: interpolate variables and escape sequences
print "Hello, $name\n";      # Output: Hello, Alice
print "Tab\tNewline\n";      # Output: Tab    Newline

# Single quotes: literal, no interpolation
print 'Hello, $name\n';      # Output: Hello, $name\n
print 'Tab\tNewline\n';      # Output: Tab\tNewline\n

String Operations

# String concatenation
my $first = "Hello";
my $second = "World";
my $combined = $first . " " . $second;  # Use dot operator
print $combined;  # Output: Hello World

# String repetition
my $dashes = "-" x 10;
print $dashes;    # Output: ----------

# String length
my $str = "Hello";
my $length = length($str);
print "Length: $length\n";  # Output: Length: 5

# Substring
my $substring = substr($str, 1, 3);  # Start at position 1, take 3 characters
print $substring;  # Output: ell

# Case conversion
print uc($str);    # Output: HELLO
print lc($str);    # Output: hello
print ucfirst($str); # Output: Hello (first letter uppercase)

Arrays

Creating Arrays

# Empty array
my @empty = ();

# Create from list
my @numbers = (1, 2, 3, 4, 5);
my @fruits = ("apple", "banana", "orange");

# Use qw for simplicity
my @colors = qw(red green blue yellow);
my @days = qw(Monday Tuesday Wednesday Thursday Friday Saturday Sunday);

# Use range
my @range1 = (1..10);       # 1 to 10
my @range2 = ('a'..'z');    # a to z
my @range3 = (1..5, 10..15); # Mixed range

Accessing Array Elements

my @numbers = (10, 20, 30, 40, 50);

# Access single element (use $ prefix)
print $numbers[0];  # Output: 10
print $numbers[2];  # Output: 30
print $numbers[-1]; # Output: 50 (last element)

# Access multiple elements (slice)
my @subset = @numbers[1, 3];   # (20, 40)
my @slice = @numbers[0..2];    # (10, 20, 30)

Array Operations

my @numbers = (1, 2, 3);

# Add elements
push @numbers, 4;              # Add at end: (1, 2, 3, 4)
unshift @numbers, 0;           # Add at beginning: (0, 1, 2, 3, 4)

# Remove elements
my $last = pop @numbers;       # Remove and return last: (0, 1, 2, 3)
my $first = shift @numbers;    # Remove and return first: (1, 2, 3)

# Array length
my $count = scalar @numbers;   # Returns 3
print $#numbers;               # Returns last index: 2

# Delete elements
delete $numbers[1];            # Delete index 1, keep position: (1, undef, 3)
splice @numbers, 1, 1;         # Delete index 1, no position: (1, 3)

# Sort
my @sorted = sort @numbers;    # Alphabetical sort
my @numeric_sorted = sort { $a <=> $b } @numbers;  # Numeric sort

# Reverse
my @reversed = reverse @numbers;  # (3, 2, 1)

Array Iteration

my @fruits = ("apple", "banana", "orange");

# Using foreach
foreach my $fruit (@fruits) {
    print "I like $fruit\n";
}

# Using for loop
for (my $i = 0; $i < @fruits; $i++) {
    print "$fruits[$i]\n";
}

# Using index
for my $index (0..$#fruits) {
    print "Index $index: $fruits[$index]\n";
}

Array to Scalar Conversion

# Array to scalar (get length)
my @array = (1, 2, 3, 4, 5);
my $count = @array;   # $count = 5

# Scalar list to array
my ($a, $b, $c) = (1, 2, 3);

# Array expansion
my @array1 = (1, 2, 3);
my @array2 = (4, 5, 6);
my @combined = (@array1, @array2);  # (1, 2, 3, 4, 5, 6)

# Array assignment to scalar (returns element count)
my $num = @array1;  # $num = 3

Hashes

Creating Hashes

# Empty hash
my %empty = ();

# Create from list (key-value pairs)
my %person = (
    name => "Alice",
    age => 25,
    city => "New York"
);

# Using commas
my %scores = (
    "math", 90,
    "english", 85,
    "science", 92
);

# Mixed style
my %data = (
    "name" => "Bob",
    age => 30,
    "city" => "London"
);

Accessing Hash Elements

my %person = (name => "Alice", age => 25);

# Access single value (use $ prefix)
print $person{name};     # Output: Alice
print $person{age};      # Output: 25

# Modify value
$person{age} = 26;

# Accessing non-existent key returns undef
print $person{city};     # Output: (empty or warning)

Hash Operations

my %person = (name => "Alice", age => 25, city => "New York");

# Get all keys
my @keys = keys %person;           # (name, age, city)
my @sorted_keys = sort keys %person;  # Sorted keys

# Get all values
my @values = values %person;       # (Alice, 25, New York)

# Get key-value pairs
my @pairs = %person;               # Expand to list
while (my ($key, $value) = each %person) {
    print "$key: $value\n";
}

# Check if key exists
if (exists $person{name}) {
    print "Name exists\n";
}

# Delete key-value pair
delete $person{age};

# Hash length
my $count = keys %person;  # Returns number of keys

# Clear hash
%person = ();

Hash Iteration

my %scores = (math => 90, english => 85, science => 92);

# Iterate over keys
foreach my $subject (keys %scores) {
    print "$subject: $scores{$subject}\n";
}

# Iterate over values
foreach my $score (values %scores) {
    print "Score: $score\n";
}

# Iterate over key-value pairs
while (my ($subject, $score) = each %scores) {
    print "$subject: $score\n";
}

Type Conversion

Automatic Conversion

Perl automatically converts between numbers and strings:

# Number to string
my $num = 42;
my $str = "Number: " . $num;   # "Number: 42"

# String to number
my $string = "123";
my $value = $string + 10;       # 133

# Non-numeric string converts to 0
my $text = "abc";
my $result = $text + 5;         # 5

Forced Conversion

# Force to number
my $str = "123abc";
my $num = int($str);        # 123
my $float = 0 + $str;       # 123

# Force to string
my $number = 456;
my $text = "" . $number;    # "456"
my $text2 = "$number";      # "456"

Special Variables

Default Variable $_

# $_ is default input and pattern search variable
$_ = "Hello World";
print;          # Print $_

# Automatically set in loops
foreach (1..5) {
    print;       # Print $_
}

# grep and map use $_
my @numbers = (1, 2, 3, 4, 5);
my @evens = grep { $_ % 2 == 0 } @numbers;  # (2, 4)
my @squared = map { $_ * $_ } @numbers;     # (1, 4, 9, 16, 25)

Practice Examples

Example 1: Shopping List

#!/usr/bin/perl
use strict;
use warnings;

my @shopping_list = qw(apple banana milk bread);
my %prices = (
    apple => 2.5,
    banana => 1.8,
    milk => 3.2,
    bread => 4.0
);

print "=== Shopping List ===\n";
foreach my $item (@shopping_list) {
    my $price = $prices{$item} || "N/A";
    printf "%-10s: \$%.2f\n", $item, $price;
}

Example 2: Student Grade Statistics

#!/usr/bin/perl
use strict;
use warnings;

my %students = (
    Alice => { math => 90, english => 85, science => 92 },
    Bob => { math => 78, english => 92, science => 88 },
    Charlie => { math => 95, english => 89, science => 97 }
);

foreach my $name (keys %students) {
    my $scores = $students{$name};
    my $sum = $scores->{math} + $scores->{english} + $scores->{science};
    my $average = $sum / 3;
    printf "%-10s: Math=%d, English=%d, Science=%d, Average=%.1f\n",
           $name, $scores->{math}, $scores->{english},
           $scores->{science}, $average;
}

Summary

In this chapter, we learned Perl's data types:

1. ✅ Scalars: Single values (numbers, strings, references)
2. ✅ Arrays: Ordered lists of values
3. ✅ Hashes: Key-value pairs
4. ✅ Type conversion
5. ✅ Special variables

Next, we will learn Perl Variables and Perl Operators.