Using grep on undefined array

[Mark Overmeer]

* Andrew Beverley (andy at andybev.com) [130813 23:24]:
> Could someone please explain to me why the following outputs an empty
> string rather than "*"?
> 
> get();
> sub get($)
> {   my $fields = shift;
>     my @fields = grep $_ ne 'domain', @$fields;
>     my $select_fields = $fields ? join(',', map { 'users.' . $_ } @fields) : '*';

I expect you would like to see the samen result when $fields is undef or
and empty ARRAY, so the answer on the first question is probably a bug:
you check for the existence of $fields, where you probably mean @fields.

      my $select_fields = @fields ? join(',', map { 'users.' . $_ } @fields) : '*';

      my $select_fields = @fields ? join(',', map "users.$_", @fields) : '*';

@fields != @$fields, which is causing the confusion.

      my @users = grep $_ ne 'domain', @$fields;
      my $select_users = @users ? join(',', map "users.$_", @users) : '*';

> I would have expected $fields to remain undefined, but it seems to be
> turning into an empty array during the grep.

Autovivification: you tell Perl that the undefined $fields is an ARRAY,
so it conforms to that.
-- 
Regards,
               MarkOv

------------------------------------------------------------------------
       Mark Overmeer MSc                                MARKOV Solutions
       Mark at Overmeer.net                          solutions at overmeer.net
http://Mark.Overmeer.net                   http://solutions.overmeer.net