Question

I’m trying to initialise a 2D array in Perl that is basically a schedule. So it looks like this:

Time     Mon   Tue   Wed   Thu   Fri
09:00     0     0     0     0     0  
10:00     0     0     0     0     0 
11:00     0     0     0     0     0 
12:00     0     0     0     0     0 
13:00     0     0     0     0     0 
14:00     0     0     0     0     0 
15:00     0     0     0     0     0 
16:00     0     0     0     0     0
17:00     0     0     0     0     0 
18:00     0     0     0     0     0 
19:00     0     0     0     0     0 
20:00     0     0     0     0     0 

I read some articles on 2D arrays in Perl but I’m still confused. What’s the easiest way to do this? And please explain a bit so I can get what’s going on.

Answer 1

Take a look at perllol. It is the best documentation for lists of lists in Perl

Essentially, each element of a Perl array can be any scalar value – a string or number, or something more exotic like a subroutine reference or a filehandle reference. To create a 2D array you want an array of arrays, so each element of the base array needs to be an array reference

The [...] construct creates an anonymous array and returns a reference to it. The anonymous part just means that it has no identifier attached to it like @data, but it exists in memory and can be accessed through its reference

The syntax for accessing an element of an array through a reference is, for instance, $array_ref->[0] for the first element. So if I write

my $array_ref = [ '09:00', 0, 0, 0, 0, 0 ];
print $array_ref->[0];

I would see 09:00 output

For your data, you can write

my @data = (
  [ '09:00', 0, 0, 0, 0, 0 ],
  [ '10:00', 0, 0, 0, 0, 0 ],
  [ '11:00', 0, 0, 0, 0, 0 ],
  [ '12:00', 0, 0, 0, 0, 0 ],
  [ '13:00', 0, 0, 0, 0, 0 ],
  [ '14:00', 0, 0, 0, 0, 0 ],
  [ '15:00', 0, 0, 0, 0, 0 ],
  [ '16:00', 0, 0, 0, 0, 0 ],
  [ '17:00', 0, 0, 0, 0, 0 ],
  [ '18:00', 0, 0, 0, 0, 0 ],
  [ '19:00', 0, 0, 0, 0, 0 ],
  [ '20:00', 0, 0, 0, 0, 0 ],
);

So $data[0] is a reference to an (anonymous) array holding the first line of data, and $data[0]->[0] is '09:00'

In addition there is a syntax facility that allows the arrow between any pair of closing and opening brackets to be omitted, so that $data[0]->[0] may be written as $data[0][0] with exactly the same meaning