hash deref confusion: Intermediate Perl

live4tech has asked for the wisdom of the Perl Monks concerning the following question:

Almost through the Intermediate Perl book and came across code I do not understand and that was not explained clearly (seems to happen to me frequently in this book!). Here is the code:

sub named { #class method
   my $self = shift->SUPER::named(@_);
   my $name = $self->name;
   my @standings = split ' ', $STANDINGS{$name} || "0 0 0 0";
   @$self{qw(wins places shows losses)} = @standings;
   $self;
}
[download]

As background: $self is a blessed hash reference. The method 'name' returns a scalar string. %STANDINGS is a hash that is associated with a DBM file.

My question: I do not understand the line @$self{qw(wins places shows losses)} = @standings;

I see that @standings becomes a list of 4 numbers as a result from splitting on spaces. But, in the next line, "@$self" is syntax for dereferencing an array reference, not a hash reference! Further, the use of the "{" after this indicates a hash, but I have never seen a hash defined in this way. Are they actually saying that you can have a list of keys ("qw(wins places shows losses)") and set the values by setting this to the list "@standings"?? Really?

PERL always seems to confound me with syntax! And when I cannot find the answer in perldocs etc. I just thorw up my hands and post it here!

THANK YOU for being there monks!

Comment on hash deref confusion: Intermediate Perl Select or Download Code

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Re: hash deref confusion: Intermediate Perl by davido (Cardinal) on Jul 30, 2012 at 06:38 UTC
Sometimes for me I find it more readable to place the inner reference in brackets, which sort of serves to remind me that it's a reference. `@{$self}{ qw( wins places shows losses ) } = ....` [download] We already know `$self` is a hashref. What happens when you put an `@` in front of a hash, and a pair of curly brackets after it? Let's look at the similar syntax, but first remove the reference indirection (and complexity): `@somehash{ qw( wins places shows losses ) } = ....` [download] That should ring a bell (you're looking at a hash slice). Dave	[reply] [d/l] [select]
Re: hash deref confusion: Intermediate Perl by BrowserUk (Patriarch) on Jul 30, 2012 at 06:41 UTC
But, in the next line, "@$self" is syntax for dereferencing an array reference, not a hash reference! The construct you refer to is called a 'hash slice' -- see perldata#slices. It assignes the 4 values derived from spliting `$STANDINGS{$name}` (or the four 0s from the constant) to the the four keys: `qw[ wins places shows losses ]` [download] . That one line is (mostly) equivalent to: `$self->{wins} = $STANDINGS[ 0 ] \|\| 0; $self->{places} = $STANDINGS[ 1 ] \|\| 0; $self->{shows} = $STANDINGS[ 2 ] \|\| 0; $self->{looses} = $STANDINGS[ 3 ] \|\| 0;` [download] With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday' Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice. The start of some sanity?	[reply] [d/l] [select]
Re: hash deref confusion: Intermediate Perl by Anonymous Monk on Jul 30, 2012 at 06:40 UTC
But, in the next line, "@$self" is syntax for dereferencing an array reference, not a hash reference! Its both; to quote myself: This is called a hash slice , and in a quirk of perl syntax uses the @ sigil instead of % my %foo; @foo{ 'hash', 'slices' } = ( 'use', 'curly braces' ); my @bar; @bar[ 1,2,3,4 ] = ( 'array', 'slices', 'use', 'square brackets' ); See also References quick reference	[reply]
Re^2: hash deref confusion: Intermediate Perl by tobyink (Canon) on Jul 30, 2012 at 07:03 UTC
I wouldn't really say it's a quirk. You're assigning a list of values to a list of hash slots, so the `@` sigil is quite appropriate. `perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'`	[reply] [d/l]
Re^3: hash deref confusion: Intermediate Perl by Anonymous Monk on Jul 30, 2012 at 08:13 UTC
I wouldn't really say it's a quirk. You're assigning a list of values to a list of hash slots, so the @ sigil is quite appropriate. Sure, from that perspective it makes sense, but IIRC TheLarry himself called this a mistake, its why perl6 always calls a %hash a `%hash{$key}` not `$hash{$key}`	[reply] [d/l] [select]
Re^4: hash deref confusion: Intermediate Perl by tobyink (Canon) on Jul 30, 2012 at 08:30 UTC
Re^5: hash deref confusion: Intermediate Perl by Anonymous Monk on Jul 30, 2012 at 08:37 UTC
Re^2: hash deref confusion: Intermediate Perl by live4tech (Sexton) on Jul 30, 2012 at 14:09 UTC
Thank you! This is a very clear and concise answer. I find it difficult to remember all the syntax in perl (also true especially of all the special symbols $....	[reply]
Re: hash deref confusion: Intermediate Perl by AnomalousMonk (Archbishop) on Jul 30, 2012 at 13:35 UTC
Are they actually saying that you can have a list of keys ("qw(wins places shows losses)") and set the values by setting this to the list "@standings"?? Really? Why not just try it and see?? A toy experimental program takes less than a minute to create and run. Really! `>perl -wMstrict -le "use Data::Dump; ;; my $self = { foo => 'bar' }; my @standings = qw(1 2 3 4); ;; @$self{ qw(wins places shows losses) } = @standings; ;; dd $self; " { foo => "bar", losses => 4, places => 2, shows => 3, wins => 1 }` [download]	[reply] [d/l]
Re^2: hash deref confusion: Intermediate Perl by live4tech (Sexton) on Jul 30, 2012 at 14:03 UTC
Of course, but you are taking this out of context. I would have had to know @$self{ qw... was a hash slice which was my real problem. I assumed the line worked, I did not understand the syntax.	[reply]


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