Perl Hash Examples

I have a friend at work who occasionally writes in Perl and so has been learning the language over quite a while, picking up new bits when he needs to write something else. I'm generally his Perl sounding-board (among other topics) and he's getting pretty good at writing things from scratch himself now, even asking questions on things I've not done with Perl, not that I'm a massively advanced Perl programmer myself.

One of the things my friend regularly needs help with and I think confuses a lot of people with Perl is with variables, and in particular hashes. People often say to me they're not sure when to use dollar, percent or at-sign in their Perl variables. I knocked together some pretty noddy code to illustrate the various ways a hash can be used in Perl code as an example for him and thought it might be useful to a wider audience so posting it here.

The source code is below with the output from running this code presented afterwards. Syntax highlighting was done courtesy of perltidy.

#!/usr/bin/perl -w
#
# Some noddy hash examples
# Note: the syntactical differences can be quite subtle
# Note: hashes don't have guaranteed ordering (run this code)

use strict;

################################################################################
# lets play with a hash
################################################################################

# define a variable as a hash with some content
my %hash = (
    'a' => 1,
    'b' => 2,
    'c' => 3
);

print "print the hash value assigned to the key 'a'\n";
print $hash{"a"}."\n\n";

print "print all of the hash values\n";
foreach my $key (keys %hash) {
  print $hash{$key}."\n";
}

print "\nprint all of the hash values in order\n";
foreach my $key (sort keys %hash) {
  print $hash{$key}."\n";
}

print "print all of the hash values with associated keys\n";
foreach my $key (keys %hash) {
  print $key . " is " . $hash{$key}."\n";
}

print "\nprint all of the hash values with associated keys in order\n";
foreach my $key (sort keys %hash) {
  print $key . " is " . $hash{$key}."\n";
}

# get rid of the hash so we're 100% sure we're not using it below!
undef %hash;


################################################################################
# lets play with a hash reference
################################################################################

# define a variable as a hash reference with some content
my $hashref = {
    'a' => 1,
    'b' => 2,
    'c' => 3
};

print "\n\nprint the hash reference value assigned to the key 'a'\n";
print $hashref->{"a"}."\n\n";

print "print all of the hash reference values\n";
foreach my $key (keys %{$hashref}) {
  print $hashref->{$key}."\n";
}

print "\nprint all of the hash reference values in order\n";
foreach my $key (sort keys %{$hashref}) {
  print $hashref->{$key}."\n";
}

print "print all of the hash reference values with associated keys\n";
foreach my $key (keys %{$hashref}) {
  print $key . " is " . $hashref->{$key}."\n";
}

print "\nprint all of the hash reference values with associated keys in order\n";
foreach my $key (sort keys %{$hashref}) {
  print $key . " is " . $hashref->{$key}."\n";
}

The output from running this code is below:

print the hash value assigned to the key 'a'
1

print all of the hash values
3
1
2

print all of the hash values in order
1
2
3
print all of the hash values with associated keys
c is 3
a is 1
b is 2

print all of the hash values with associated keys in order
a is 1
b is 2
c is 3


print the hash reference value assigned to the key 'a'
1

print all of the hash reference values
3
1
2

print all of the hash reference values in order
1
2
3
print all of the hash reference values with associated keys
c is 3
a is 1
b is 2

print all of the hash reference values with associated keys in order
a is 1
b is 2
c is 3