Lnation

Learning Perl - Conditional Statements

So far we have covered basic variables in Perl, today we are going to look at how to use these variables in conditional statements.

A conditional statement in programming is a statement that executes a block of code based on whether a condition is true or false. In Perl, we can use 'if', 'elsif', 'else' and 'unless' to create conditional statements.

The most basic conditional in Perl is the 'if' statement. It allows you to execute a block of code only if a certain condition is true. You can then chain multiple conditions using 'elsif' and provide a default action using 'else'.

When you want to check a condition, you can use comparison operators like '==' for numeric comparison or 'eq' for string comparison, essentially the code that is within the condition must return either true or false.

You can use the following table as a reference of operators that can be used in conditions when comparing scalars:

Operator	Description	Example
==	Numeric equality	$a == $b
!=	Numeric inequality	$a != $b
<	Less than	$a < $b
>	Greater than	$a > $b
<=	Less than or equal to	$a <= $b
>=	Greater than or equal to	$a >= $b
eq	String equality	$a eq $b
ne	String inequality	$a ne $b
lt	String less than	$a lt $b
gt	String greater than	$a gt $b
le	String less or equal	$a le $b
ge	String greater or equal	$a ge $b
defined	Check if variable is defined	defined $var
exists	Check if hash key exists	exists $hash{$key}
&&	Logical AND	$a && $b
and	Logical AND (lower precedence)	$a and $b

//	Defined or	$a // $b
or	Logical OR (lower precedence)	$a or $b
!	Logical NOT	!$a

Today we will create a few simple Perl scripts that checks the value of a command line input and prints different messages based on its value. First create a file named 'conditional.pl' and add the following code:

#!/usr/bin/perl
use strict;
use warnings;
my $value = shift;
if ($value < 5) {
    print "Value is less than 5\n";
} elsif ($value == 10) {
    print "Value is equal to 10\n";
} else {
    print "Value is greater than 5 and not equal to 10\n";
}

Save the file and run it using the command:

perl conditional.pl 10

The output should be:

Value is equal to 10

We have created a variable '$value' that takes an input from the command line, we receive the input by using 'shift' to remove the first argument from the stack. We will talk more about the stack in a future post. The script then checks if the value is less than 5, equal to 10, or greater than 5 and not equal to 10. We use the '<' less than operator to check if the value is less than 5, '==' to check if it is equal to 10, and the else statement to handle all other cases.

You can also use string comparisons in a similar way. For example, if you want to check if a string variable is equal to another string, you can use the eq operator. Here’s an example:

#!/usr/bin/perl
use strict;
use warnings;
my $name = shift;
if ($name eq "Alice") {
    print "Hello, Alice!\n";
} elsif ($name eq "Bob") {
    print "Hello, Bob!\n";
} else {
    print "Hello, stranger!\n";
}

Save this code in a file named 'string_conditional.pl' and run it with:

perl string_conditional.pl Alice

You should see the output:

Hello, Alice!

If you change the input to "Bob" or any other name, the script will respond accordingly.

You can also use logical operators to combine conditions. For example, if you want to check if a number is between two values, you can do something like this:

#!/usr/bin/perl
use strict;
use warnings;
my $number = shift;
if ($number >= 1 && $number <= 10) {
    print "Number is between 1 and 10\n";
} elsif ($number > 10 && $number <= 20) {
    print "Number is between 11 and 20\n";
} else {
    print "Number is outside the range of 1 to 20\n";
}

Save this code in a file named 'range_conditional.pl' and run it with:

perl range_conditional.pl 15

You should see the output:

Number is between 11 and 20

Another conditional statement that can be used in Perl is 'unless', which is the opposite of 'if'. It executes a block of code only if the condition is false. Here’s an example:

#!/usr/bin/perl
use strict;
use warnings;
my $age = shift;
unless ($age >= 18) {
    print "You are not old enough to vote.\n";
} else {
    print "You are old enough to vote.\n";
}

Save this code in a file named 'unless_conditional.pl' and run it with:

perl unless_conditional.pl 16

You should see the output:

You are old enough to vote.

You can also chain elsif and else statements to handle multiple conditions. Here’s a more complex example:

#!/usr/bin/perl
use strict;
use warnings;
my $score = shift;
unless(defined $score) {
    print "No score provided.\n";
} elsif ($score < 50) {
    print "You failed the test.\n";
} elsif ($score < 75) {
    print "You passed the test.\n";
} else {
    print "You excelled in the test!\n";
}

Save this code in a file named defined_conditional.pl and run it with:

perl defined_conditional.pl 85

You should see the output:

You excelled in the test!

That concludes our post today, we have covered the basics of conditional statements in Perl. In the next post we will look at loops, which will allow us to repeat actions based on conditions.