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Perl — 3

Himashi Karunathilake
10 min readAug 16, 2023

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Image from https://www.vectorlogo.zone/logos/perl/perl-ar21.svg
Link to Part 1: https://himashikarunathilake.medium.com/perl-1-5a5f4ec8c251
Link to Part 2: https://himashikarunathilake.medium.com/perl-2-12f31be96028

Given below is the main.pl file that will be used to run all the sub files in this section:

#!/usr/bin/perl

# The third program in Perl.

use strict;
use warnings;
use feature "say";

say "";

say "*************** RUNNING THE INPUT_STATEMENT.PL FILE ***************";
system("perl input_statement.pl");
say "__________________________________________________________________________________________";
say "";

say "*************** RUNNING THE CONDITIONS.PL FILE ***************";
system("perl conditions.pl");
say "__________________________________________________________________________________________";
say "";

say "*************** RUNNING THE LOOPS.PL FILE ***************";
system("perl loops.pl");
say "__________________________________________________________________________________________";
say "";

say "*************** RUNNING THE SPECIAL_STATEMENTS.PL FILE ***************";
system("perl special_statements.pl");
say "__________________________________________________________________________________________";
say "";

The Input Statement

To read an input from a user or a file in Perl, we can use the diamond operator “<>” as it reads inputs line by line.

#!/usr/bin/perl

use strict;
use warnings;
use feature "say";

print "Enter your first name: ";

# Read the input from the user and assign it to a variable
my $first_name = <STDIN>;

# Remove the newline character from the input
chomp $first_name;

say "Hi $first_name!";
Obtaining a user input
#!/usr/bin/perl

use strict;
use warnings;
use feature "say";

print "Enter your first name: ";

# Read the input from the user and assign it to a variable
my $first_name = <STDIN>;

# Remove the newline character from the input
chomp $first_name;

say "Hi $first_name!";

say "";

# Obtain the name of the file to be read
print "Enter the name of the file to be read (with the file extension): ";
my $file_name = <STDIN>;
chomp $file_name;

# Read input from the file
open my $file, '<', $file_name or die "Error! Cannot open file: $!";
my $i = 1;
while (<$file>) {
    chomp;
    print "Line $i of file: $_\n";
    $i++;
}

close $file;
input_file.txt
Reading an input from a file

Condition Statements

if Statement

The following piece of code checks whether the user provides a last name. If the input is given as -1, the program will exit that particular code block.

#!/usr/bin/perl

use strict;
use warnings;
use feature "say";

# if statement
print "Enter your last name (-1 to exit): ";
my $last_name = <STDIN>;
chomp $last_name;

if ($last_name ne "-1") {
    say "$last_name\'s Conditions Program";
}
The program executes when the last name is provided
The program exits that code block if the input is given as -1

if-else Statement

The following piece of code checks whether the user is an adult based on their input.

#!/usr/bin/perl

use strict;
use warnings;
use feature "say";

# if statement
print "Enter your last name (-1 to exit): ";
my $last_name = <STDIN>;
chomp $last_name;

if ($last_name ne "-1") {
    say "$last_name\'s Conditions Program";
}

say "";

# if-else statement
print "Enter your age: ";
my $age = <STDIN>;
chomp $age;

if ($age >= 18) {
    say "You are an adult!";
} else {
    say "You are a child!";
}
Output when the input is 16
Output when the input is 18
Output when the input is 29
If required,you can also follow the following format to have multiple 
conditions within an if-else statement:

if (condition1) {
  # Statements to be executed if the condition1 is met
} elsif (condition2) {
  # Statements to be executed if the condition2 is met
} elsif (condition3) {
  # Statements to be executed if the condition3 is met
} ...
...
...
} else {
  # Statements to be executed if none of the above conditions are met
}

Conditional Statements with the Ternary Operator

The ternary operator allows you to write compact conditional statements.

condition ? expression_if_true : expression_if_false;
#!/usr/bin/perl

use strict;
use warnings;
use feature "say";

# if statement
print "Enter your last name (-1 to exit): ";
my $last_name = <STDIN>;
chomp $last_name;

if ($last_name ne "-1") {
    say "$last_name\'s Conditions Program";
}

say "";

# if-else statement
print "Enter your age: ";
my $age = <STDIN>;
chomp $age;

if ($age >= 18) {
    say "You are an adult!";
} else {
    say "You are a child!";
}

say "";

# Terenary operator
print "Enter the current temperature: ";
my $temperature = <STDIN>;
chomp $temperature;

my $weather = ($temperature > 25) ? "warm" : "cold";
say "The current weather is $weather!";
Output when the input is 21
Output when the input is 25
Output when the input is 31

unless Statement

As discussed above, an if statement will be executed if the given condition is met. However, we can use an unless statement to be executed if the given condition is not met.

#!/usr/bin/perl

use strict;
use warnings;
use feature "say";

# if statement
print "Enter your last name (-1 to exit): ";
my $last_name = <STDIN>;
chomp $last_name;

if ($last_name ne "-1") {
    say "$last_name\'s Conditions Program";
}

say "";

# if-else statement
print "Enter your age: ";
my $age = <STDIN>;
chomp $age;

if ($age >= 18) {
    say "You are an adult!";
} else {
    say "You are a child!";
}

say "";

# Terenary operator
print "Enter the current temperature: ";
my $temperature = <STDIN>;
chomp $temperature;

my $weather = ($temperature > 25) ? "warm" : "cold";
say "The current weather is $weather!";

say "";

# unless statement
print "Enter your city (-1 to exit): ";
my $city = <STDIN>;
chomp $city;

unless ($city eq "-1") {
    say "$city is the most beautiful city in the world!";
}
Output when the input is New York
Output when the input is -1
The unless statement can be also modified as an unless-else statement as 
follows:

unless (condition) {
  # Statements to be executed if the condition is not met
} else {
  # Statements to be executed if the condition is not met
}

It can be further modified into a unless-elsif-else statement as follows:

unless (condition1) {
  # Statements to be executed if the condition1 is not met
} elsif (condition2) {
  # Statements to be executed if the condition2 is met
} elsif (condition3) {
  # Statements to be executed if the condition3 is met
} ...
...
...
} else {
  # Statements to be executed if none of the above conditions are met
}

Loops

for Loop

Repeatedly executes a block of code until a given condition is met.

#!/usr/bin/perl

use strict;
use warnings;
use feature "say";

# for loop
say "Printing numbers less than 5..........";
for (my $i = 0; $i < 5; $i++) {
    print "$i\t";
}

say "";
Using a for loop to print numbers less than 5

while Loop

The while loop in Perl repeatedly executes a block of code for as long as the given condition is true.

#!/usr/bin/perl

use strict;
use warnings;
use feature "say";

# for loop
say "Printing numbers less than 5..........";
for (my $i = 0; $i < 5; $i++) {
    print "$i\t";
}

say "";
say "";
say "";

# while loop
print "Please enter a number less than or equal to 10: ";
my $number = <STDIN>;
chomp $number;

if ($number <= 10) {
    say "Printing numbers less than $number..........";

    my $j = 0;
    while ($j < $number) {
        print "$j\t";
        $j++;
    }
} else {
    say "ERROR! The number $number is greater than 10.\n";
}

say "";
Printing numbers less than a user provided value using a while loop

until Loop

The until loop in Perl is similar to the while loop but continues executing for as long as the given condition is false.

#!/usr/bin/perl

use strict;
use warnings;
use feature "say";

# for loop
say "Printing numbers less than 5..........";
for (my $i = 0; $i < 5; $i++) {
    print "$i\t";
}

say "";
say "";
say "";

# while loop
print "Please enter a number less than or equal to 10: ";
my $number = <STDIN>;
chomp $number;

if ($number <= 10) {
    say "Printing numbers less than $number..........";

    my $j = 0;
    while ($j < $number) {
        print "$j\t";
        $j++;
    }
} else {
    say "ERROR! The number $number is greater than 10.\n";
}

say "";
say "";
say "";

# until loop
say "Printing numbers from 1 to 10 in the descending order..........";
my $k = 10;
until ($k == 0) {
    print "$k\t";
    $k--;
}

say "";
Printing numbers from 1 to 10 in the descending order using an until loop

foreach Loop

The foreach loop is used to iterate over elements of an array or list.

#!/usr/bin/perl

use strict;
use warnings;
use feature "say";

# for loop
say "Printing numbers less than 5..........";
for (my $i = 0; $i < 5; $i++) {
    print "$i\t";
}

say "";
say "";
say "";

# while loop
print "Please enter a number less than or equal to 10: ";
my $number = <STDIN>;
chomp $number;

if ($number <= 10) {
    say "Printing numbers less than $number..........";

    my $j = 0;
    while ($j < $number) {
        print "$j\t";
        $j++;
    }
} else {
    say "ERROR! The number $number is greater than 10.\n";
}

say "";
say "";
say "";

# until loop
say "Printing numbers from 1 to 10 in the descending order..........";
my $k = 10;
until ($k == 0) {
    print "$k\t";
    $k--;
}

say "";
say "";
say "";

# foreach loop
my @names = ("Dwight", "Jim", "Pam", "Michael", "Oscar", "Angela", "Kevin", "Andy", "Ryan", "Toby");
say "Printing the names..........";
foreach my $name (@names) {
    print "$name\t";
}

say "";
Printing the elements of an array using a foreach loop

do-while Loop

A do-while loop is similar to the while loop, but in this case, the do-while loop executes at least once since the condition is checked only after the first execution.

#!/usr/bin/perl

use strict;
use warnings;
use feature "say";

# for loop
say "Printing numbers less than 5..........";
for (my $i = 0; $i < 5; $i++) {
    print "$i\t";
}

say "";
say "";
say "";

# while loop
print "Please enter a number less than or equal to 10: ";
my $number = <STDIN>;
chomp $number;

if ($number <= 10) {
    say "Printing numbers less than $number..........";

    my $j = 0;
    while ($j < $number) {
        print "$j\t";
        $j++;
    }
} else {
    say "ERROR! The number $number is greater than 10.\n";
}

say "";
say "";
say "";

# until loop
say "Printing numbers from 1 to 10 in the descending order..........";
my $k = 10;
until ($k == 0) {
    print "$k\t";
    $k--;
}

say "";
say "";
say "";

# foreach loop
my @names = ("Dwight", "Jim", "Pam", "Michael", "Oscar", "Angela", "Kevin", "Andy", "Ryan", "Toby");
say "Printing the names..........";
foreach my $name (@names) {
    print "$name\t";
}

say "";
say "";
say "";

# do-while loop
say "Printing numbers from 1 to 10 in the ascending order..........";
my $l = 1;
do {
    print "$l\t";
    $l++;
} while ($l <= 10);

say "";
Printing numbers from 1 to 10 in the ascending order using a do-while loop

Special Statements

last Statement

To achieve the equivalent of a “break” statement, we can use the “last” statement to terminate the innermost loop.

#!/usr/bin/perl

use strict;
use warnings;
use feature "say";

# last statement
say "Printing numbers till 13 is met..........";
for (my $i = 0; $i < 20; $i++) {
    if ($i == 13) {
        last;
    }
    print "$i\t";
}

say "";
Printing numbers less than or equal to 20, until 13 is met (the loop breaks at 13)

next Statement

To achieve the equivalent of a “continue” statement, we can use the “next” statement to skip the remaining iterations of the current iteration of a loop and proceed to the next iteration.

#!/usr/bin/perl

use strict;
use warnings;
use feature "say";

# last statement
say "Printing numbers till 13 is met..........";
for (my $i = 0; $i < 20; $i++) {
    if ($i == 13) {
        last;
    }
    print "$i\t";
}

say "";
say "";
say "";

# next statement
say "Printing odd numbers less than 20..........";
for (my $j = 0; $j < 20; $j++) {
    if ($j % 2 == 0) {
        next;
    }
    print "$j\t";
}

say "";
Printing odd numbers less than 20 using a next statement that skips over the even numbers

return Statement

The return statement in Perl is used to terminate the execution of a subroutine (function) and return a value if expected.

#!/usr/bin/perl

use strict;
use warnings;
use feature "say";

sub add {
    my ($num1, $num2) = @_;
    return $num1 + $num2;
}

# last statement
say "Printing numbers till 13 is met..........";
for (my $i = 0; $i < 20; $i++) {
    if ($i == 13) {
        last;
    }
    print "$i\t";
}

say "";
say "";
say "";

# next statement
say "Printing odd numbers less than 20..........";
for (my $j = 0; $j < 20; $j++) {
    if ($j % 2 == 0) {
        next;
    }
    print "$j\t";
}

say "";
say "";
say "";

# return statement
say "Proceeding to add two numbers..........";
print "Enter the first number: ";
my $num1 = <STDIN>;
chomp $num1;
print "Enter the second number: ";
my $num2 = <STDIN>;
chomp $num2;

my $sum = add($num1, $num2);

say "The sum of the two numbers are: $sum";
Printing the sum of two user provided values using a subroutine that returns the sum of the two values

You can access the source code at: Perl/Part-3 at master · Himashi-Karunathilake/Perl (github.com).

Link to Part 4: Perl — 4. Given below is the main.pl file that… | by Himashi Karunathilake | Aug, 2023 | Medium

Perl
Introduction
Programming
Beginner
Scripting

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Himashi Karunathilake

Written by Himashi Karunathilake

7 Followers

I am a cybersecurity enthusiast and writer with a passion for demystifying complex topics. Join me as I explore the ever-evolving world of cybersecurity!

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