unshifting by reference

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

Hi

I got this var $this->{M} which is an array & $this->{S} which is a scalar.
(those are part of a bigger structure which I can't alter)
The problem is when I try unshift($this->{M},$this->{S});
it says Type of arg 1 to unshift must be array (not hash element)
it says the same even when I avoid using strict.
so I tried unshift(@this->{M},$this->{S});
and it said Global symbol "@this" requires explicit package name
so I tried unshift(@($this->{M}),$this->{S});
and it said Scalar found where operator expected
yeah I'm sure I'm missing something really oblivious,
but I'm getting tired of the sight of my own tail.

your thoughts ?
Thx

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Re: unshifting by reference by Eliya (Vicar) on Dec 16, 2011 at 00:29 UTC
`unshift(@($this->{Memory}),$this->{Senses});` [download] To dereference the array ref, you need curlies, not parentheses, i.e. `@{$this->{Memory}}` `my $this = { Memory => ["foo"], Senses => "bar", }; unshift @{$this->{Memory}}, $this->{Senses}; use Data::Dumper; print Dumper $this; __END__ $VAR1 = { 'Senses' => 'bar', 'Memory' => [ 'bar', 'foo' ] };` [download]	[reply] [d/l] [select]
Re^2: unshifting by reference by palkia (Monk) on Dec 16, 2011 at 00:43 UTC
Thx it works (obvious it was ^^). But I don't et it, I thought that the "->" dereferences, doesn't it ? when I did `print $this->{N}[0]{Value};` it worked ok, so whats the difference ? Why didn't I needed to dereference there ? and whats "Dumper" is this really required ? (I prefer to avoid module dependencies) Thx again	[reply] [d/l]
Re^3: unshifting by reference by Eliya (Vicar) on Dec 16, 2011 at 00:51 UTC
The thing is that `unshift` needs an array (syntactically), not an array reference. And the way to get an (entire) array from a reference is to say `@$aref`, or `@{ $something->{that}{returns}[1]{aref} }` for more complex expressions. To address a single element of the array, you'd say `$aref->[$index]`. (The Data::Dumper module is to easily print out complex data structures. I just used it for demo purposes — you of course don't need it to unshift the value.)	[reply] [d/l] [select]
Re^3: unshifting by reference by ikegami (Patriarch) on Dec 16, 2011 at 01:36 UTC
`->` is dereferencing a hash that has nothing to do with your question. Your question is about dereferencing the array referenced by `$this->{N}`.	[reply] [d/l] [select]
Re^3: unshifting by reference by ikegami (Patriarch) on Dec 16, 2011 at 01:37 UTC
`->` is dereferencing a hash that has nothing to do with your question. Your question is about dereferencing the array referenced by `$this->{N}`.	[reply] [d/l] [select]
Re: unshifting by reference by kejohm (Hermit) on Dec 16, 2011 at 04:56 UTC
On a side note, as of Perl v5.14, `shift`, `unshift`, `push`, and `pop` can now take a reference to an unblessed array as their first argument, which will be automatically dereferenced, although it is considered experimental.	[reply] [d/l] [select]