Creating arrays dynamically in Perl

Question

I want to create arrays dynamically based on the user input. For example, if the user gives input as 3 then three arrays should be created with the name @message1

Answer 1

Creating new named arrays dynamically is almost never a good idea. Mark Dominus, author of the enlightening book Higher-Order Perl, has written a three-part series detailing the pitfalls.

You have names in mind for these arrays, so put them in a hash:

sub create_arrays {
  my($where,$n) = @_;

  for (1 .. $n) {
    $where->{"message$_"} = [];
  }
}

For a quick example that shows the structure, the code below

my $n = @ARGV ? shift : 3;

my %hash;
create_arrays \%hash, $n;

use Data::Dumper;
$Data::Dumper::Indent = $Data::Dumper::Terse = 1;
print Dumper \%hash;

outputs

$ ./prog.pl
{
  'message2' => [],
  'message3' => [],
  'message1' => []
}

Specifying a different number of arrays, we get

$ ./prog.pl 7
{
  'message2' => [],
  'message6' => [],
  'message5' => [],
  'message4' => [],
  'message3' => [],
  'message1' => [],
  'message7' => []
}

The order of the keys looks funny because they're inside a hash, an unordered data structure.

Recall that [] creates a reference to a new anonymous array, so, for example, to add values to message2, you'd write

push @{ $hash{"message2"} }, "Hello!";

To print it, you'd write

print $hash{"message2"}[0], "\n";

Maybe instead you want to know how long all the arrays are:

foreach my $i (1 .. $n) {
  print "message$i: ", scalar @{ $hash{"message$i"} }, "\n";
}

For more details on how to use references in Perl, see the following documentation:

• Mark's very short tutorial about references, or perldoc perlreftut
• Perl references and nested data structures, or perldoc perlref
• Perl Data Structures Cookbook, or perldoc perldsc