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Moose and arrays

waldm
New User

May 30, 2014, 5:17 AM

Post #1 of 3 (5149 views)

Moose and arrays	Can't Post

Hi,

I have previous experience in other programming languages (Java, C#, Python), and just started learning Perl a couple of days ago.

I'm trying to write a basic calculator, and so far have the following code:

main.pl

Code

  
use Calculator; 
 
 
 
my $calculator = Calculator->new(); 
 
$calculator->add(1, 2); 
 
$calculator->add(3, 2); 
 
my $calculations = $calculator->calculations; 
 
 
 
print "The calculator has [@$calculations].\n";

Calculation.pm

Code

package Calculation; 
 
use Moose; 
 
 
 
has 'expression' => ( 
 
	is       => 'rw', 
 
	isa      => 'Str', 
 
	required => 1, 
 
	); 
 
 
 
has 'result' => ( 
 
	is       => 'rw', 
 
	isa      => 'Num', 
 
	required => 1, 
 
	); 
 
 
 
1;

Calculator.pm

Code

package Calculator; 
 
use Moose; 
 
use Calculation; 
 
 
 
 
 
has 'calculations' => ( 
 
	is  => 'rw', 
 
	isa => 'ArrayRef[Calculation]',	 
 
	); 
 
 
 
sub add { 
 
	my $self = shift;	 
 
	my @calculations = $self->calculations; 
 
	 
 
	my $x = $_[0];	 
 
	my $y = $_[1];		 
 
	 
 
	my $calculation = Calculation->new( 
 
		expression => "$x+$y", 
 
		result     => $x + $y); 
 
 
 
	push(@calculations, $calculation);	 
 
	$self->calculations(\@calculations);	 
 
} 
 
 
 
1;

but the following error occurs:

Code

Attribute (calculations) does not pass the type constraint because: Validation failed for 'ArrayRef[Calculation]' with value [ undef, Calculation={ expression: "1+2", result: 3 } ] at accessor Calculator::calculations (defined at Calculator.pm line 6) line 4. 
	Calculator::calculations('Calculator=HASH(0x198ebd0)', 'ARRAY(0x25bc740)') called at Calculator.pm line 23 
	Calculator::add('Calculator=HASH(0x198ebd0)', 1, 2) called at main.pl line 4

If I change the line

Code

push(@calculations, $calculation);

Code

$calculations[0] = $calculation;

, then it works, but obviously this will only allow me to store one calculation.

Does anyone know what the problem is here?

Thanks

p.s. I tried to use the [perl] tag instead of the code tag, but it didn't appear to work.

waldm
New User

May 30, 2014, 6:04 AM

Post #2 of 3 (5144 views)

Re: [waldm] Moose and arrays [In reply to]	Can't Post

Changing Calculator.pm to be:

Code

package Calculator;   
 
use Moose;   
 
use Calculation;  
 
  
 
  
 
  
 
has 'calculations' => (   
 
	is  => 'rw',   
 
	isa => 'ArrayRef[Calculation]',	   
 
	default => sub {my @c = (); return \@c;}, 
 
	);  
 
  
 
  
 
  
 
sub add {   
 
	my $self = shift;		 
 
	   
 
	my $x = $_[0];	   
 
	my $y = $_[1];	  
 
  
 
	my $calculation = Calculation->new(   
 
		expression => "$x+$y",   
 
		result     => $x + $y);  
 
 	 
 
	push(@{$self->calculations}, $calculation);	  	 
 
}   
 
  
 
1;

fixes it, with the main change being the line:

Code

default => sub {my @c = (); return \@c;},

Is this the correct way to initialise an array in a class?

Zhris
Enthusiast

May 30, 2014, 3:25 PM

Post #3 of 3 (5132 views)

Re: [waldm] Moose and arrays [In reply to]	Can't Post

It would be nicer to return an anonymous array reference instead:

Code

my @c = (); return \@c; 
return [];

Chris

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