Programonic

Before you start

Perl arrays.

Define a sub that reads its arguments from @_.
Predict the sample output and explain the state changes.
Complete the practice task and check the boundary cases.

The concept

A subroutine is declared with sub. All of its arguments arrive in the special array @_, and my @values = @_ copies them into a named variable so the body is easier to read.

return sends back a value. Dividing by @values uses the array in numeric context, which is its length. A subroutine defined before it is used can be called with parentheses and a list of arguments.

Code example

Perl

use strict;
use warnings;

sub average {
    my @values = @_;
    my $total = 0;
    $total += $_ for @values;
    return $total / @values;
}

print average(82, 91, 76, 95), "\n";

Output

Example details

Follow the code

Open the walkthrough to inspect the example one step at a time.

Show guided walkthrough

Guided walkthroughReady

1use strict;

2use warnings;

4sub average {

5 my @values = @_;

6 my $total = 0;

7 $total += $_ for @values;

8 return $total / @values;

9}

11print average(82, 91, 76, 95), "\n";

Follow the example

Start to inspect the source, values, and output. These prepared steps explain the published sample; grouped loop steps describe multiple iterations.

View the complete dry-run table

Prepared trace of the published program. Some rows group repeated iterations.
What happens	State after step	Output so far
Define a subroutine named average.	`—`	—
Copy the four arguments into @values.	`@values = (82, 91, 76, 95)`	—
Sum the values.	`$total = 344`	—
344 / 4 is 86, printed without a decimal point.	`return = 86`	86

Behind the scenes

How Perl processes this code

Perl compiles the script internally before executing it. strict and warnings help identify mistakes.

Cost and common mistakes

O(n) time and space for the copy.

Dividing by an empty @values dies with 'Illegal division by zero'.
Without my, variables become globals, which use strict reports as an error.

Try it yourself

Predict average(1, 2).

Reveal answer and reasoning

3 / 2 is 1.5, so it prints 1.5.

Perl Subroutines