CODEWITHSUNDEEP
perl
user785099
user785099

Reputation: 5563

issues on a segment of Perl code

I am starting to learn Perl, and have questions on the following segment of Perl code.

I know "my" is used to define a local variable, and "shift" is used for getting the head element from an array. What confused me is that where does array come from in the following code segment.

Besides, what does my @positives = keys %{$lab1->{$cate1}}stand for?

preData($cate1, $lab1)

sub preData
{
  my $cate1 = shift;
  my $lab1 = shift;

  my @positives = keys %{$lab1->{$cate1}};
}

Upvotes: 2

Views: 129

Answers (3)

Alex Howansky
Alex Howansky

Reputation: 53636

@ denotes an array and % denotes a hash. So this statement:

my @x = keys %y;

Means take the list of keys from the hash y and assign them to the array x. I.e., if %y is:

one => 1,
two => 2,
three => 3

Then @x would contain:

one, two, three

Upvotes: 2

David Brigada
David Brigada

Reputation: 594

When you call a subroutine in Perl, the arguments to that subroutine are passed to it in the @_ array. Calling shift without any parameters will take its arguments from the @_ array. So this code is taking $cate1 from the first parameter to preData and $lab1 from the second parameter to preData.

Upvotes: 2

CanSpice
CanSpice

Reputation: 35828

$lab1 is a hash reference containing other hash references. $cate1 is some kind of category key (I'm guessing).

$lab1->{$cate1} is a hash reference. When you dereference it by putting %{ ... } around it, you get a hash back. This hash is then passed to the keys() function, which returns a list of the keys in that hash. Thus @positives is an array of the keys in the hash referenced by the $lab1->{$cate1} hash reference.

Edit: When dealing with these sorts of nested structures, you may find it easier to understand what's going on by seeing a representation of the data. At the top of your script, add use Data::Dumper. Then between the my $lab1... and my @positives... lines, add:

print Dumper($lab1);
print Dumper($lab1->{$cate1});

And after you set the @positives array, add:

print Dumper(\@positives);

This should allow you to better visualize the data and hopefully get a better understanding of Perl structures.

Upvotes: 5

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