Using Dereferencing

catfish1116 has asked for the wisdom of the Perl Monks concerning the following question:

Trying out an exercise that uses dereferencing Below is code and error

#! /usr/bin/perl
use v5.16.3;
use strict;
use warnings;

## 7/31/19
## program uses derefence to simplify subroutine from previous program


   sub check_required_items {
     my ($who, $items) = @_;
     my %whos_items = map { $_, 1 } @$items; # the rest are the person
+'s items

     my @required = qw(preserver sunscreen water_bottle jacket);

     for my $item (@required) {
       unless ( $whos_items{$item} ) { # not found in list 
         print "$who is missing $item, \n";
       }
     }
   }


   my @gilligan = qw(red_shirt hat lucky_socks water_bottle);
   check_required_items('gilligan', @gilligan);
   my @skipper = qw(blue_shirt hat jacket preserver sunscreen);
   my @professor = qw(sunscreen water_bottle slide_rule batteries radi
+o);
   check_required_items('skipper', @skipper);
   check_required_items('professor', @professor);

Can't use string ("red_shirt") as an ARRAY ref while "strict refs" in 
+use at ./Page_42 line 12.
[download]

TIA The Catfish

Comment on Using Dereferencing Download Code

Replies are listed 'Best First'.
Re: Using Dereferencing by Fletch (Bishop) on Jul 31, 2019 at 17:50 UTC
Presuming your question is the implied "Why am I getting this errror?" . . . When you call `check_required_items( 'foo', @bar )` the array `@bar` is expanded out into the contained items. The sub however expects it to be a reference to an array and it's telling you it can't use the string "red_shirt" as a reference to the array. You want to use a backslash in front of the array name to pass a reference to it: `check_required_items( 'gilligan', \@gilligan )`. See perlreftut for more details. The cake is a lie. The cake is a lie. The cake is a lie.	[reply] [d/l] [select]
Re: Using Dereferencing by soonix (Chancellor) on Jul 31, 2019 at 18:19 UTC
`my ($who, $items) = @_;` You seem to expect an array ref in `$items`, actually, it will be just the second argument (first item of e.g. `@gilligan`), the rest will be discarded. Why not `my ($who, @items) = @_;` [download] then you could `map ... @items` without needing to dereference.	[reply] [d/l] [select]
Re: Using Dereferencing by BillKSmith (Monsignor) on Jul 31, 2019 at 18:26 UTC
Your use of the name Page_42 number suggests that you are using the book "Intermediate Perl". You can find a correct example of your function call on page 41. You omitted the "take a reference to" operator (also known as the 'blackslash" operator in Perl documentation perlref). It is explained on pages 38 and 39. Bill	[reply]
Re: Using Dereferencing by AnomalousMonk (Archbishop) on Jul 31, 2019 at 19:25 UTC
Others have pinpointed the problem of not taking and passing an array reference. Just for grins, run the OPed code without strictures asserted, e.g., `# use v5.16.3;` `# use strict;` (the `use v5.16.3;` statement has to be commented out because IIRC use-ing Perl version 5.16+ automatically asserts strictures), and see what happens. Note that you can still assert warnings. Is there necessarily any hint that something might be going wrong? Give a man a fish: `<%-{-{-{-<`	[reply] [d/l] [select]
Re: Using Dereferencing by TieUpYourCamel (Scribe) on Dec 12, 2019 at 21:28 UTC
I think the following is cleaner. There's no need to create a separate array. Just shift off the name and map the rest. `my $who = shift; my %whos_items = map { $_, 1 } @_; # the rest are the person's items` [download]	[reply] [d/l]