Programonic

Before you start

Perl scalars.

Store a list in an @array and loop over it.
Predict the sample output and explain the state changes.
Complete the practice task and check the boundary cases.

The concept

A Perl array variable starts with @. push adds elements to the end. Using an array in scalar context, as scalar(@scores) does, gives its length.

The statement modifier $total += $_ for @scores runs the addition once for each element, with $_ holding the current one. my declares the variables lexically, and use strict makes the compiler enforce it.

Code example

Perl

use strict;
use warnings;

my @scores = (82, 91, 76);
push @scores, 95;
my $total = 0;
$total += $_ for @scores;
print scalar(@scores), " ", $total, "\n";

Output

4 344

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;

4my @scores = (82, 91, 76);

5push @scores, 95;

6my $total = 0;

7$total += $_ for @scores;

8print scalar(@scores), " ", $total, "\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
An array of three numbers.	`@scores = (82, 91, 76)`	—
Append a fourth element.	`@scores = (82, 91, 76, 95)`	—
Add each element to the total.	`$total = 344`	—
Print the length and the sum.	`$total = 344`	4 344

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

push is amortized O(1); the loop is O(n).

print @scores without separators glues the elements together; join them explicitly.
The last index is $#scores, which is one less than the length.

Try it yourself

Print the first element.

Reveal answer and reasoning

print $scores[0]; uses $ because a single element is a scalar. It prints 82.

Perl Arrays