Programonic

Before you start

Perl arrays.

Map keys to values and print them in a stable order.
Predict the sample output and explain the state changes.
Complete the practice task and check the boundary cases.

The concept

A hash variable starts with %. It stores key-value pairs, and a single value is read with $scores{Ada}, with a $ and curly braces. Adding a key is just assigning to it.

Perl does not promise any order for hash keys, and the order can change between runs. keys %scores returns the keys, and sort puts them in a predictable alphabetical order before the loop.

Code example

Perl

use strict;
use warnings;

my %scores = (Ada => 91, Grace => 88);
$scores{Linus} = 79;
for my $name (sort keys %scores) {
    print "$name: $scores{$name}\n";
}

Output

Ada: 91
Grace: 88
Linus: 79

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 = (Ada => 91, Grace => 88);

5$scores{Linus} = 79;

6for my $name (sort keys %scores) {

7 print "$name: $scores{$name}\n";

8}

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
A hash with two pairs; => quotes the bare words.	`%scores has 2 pairs`	—
Add a third key.	`%scores has 3 pairs`	—
Visit the keys alphabetically: Ada, Grace, Linus.	`%scores has 3 pairs`	—
Print each pair.	`%scores has 3 pairs`	Ada: 91 Grace: 88 Linus: 79

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

Lookup is O(1) on average; the sorted loop is O(k log k) for k keys.

Without sort, the key order is unspecified.
Reading a missing key returns undef, which warns when printed under use warnings.

Try it yourself

Remove Grace before the loop.

Reveal answer and reasoning

Use delete $scores{Grace}; only Ada and Linus are printed.

Perl Hashes