Complicated Hash Construction.

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

Why is it that

my %hash;
@hash{@keyarray}=@valuearray;
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is ok, but combining the two lines

my @hash{@keyarray}=@valuearray;
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is a syntax error? I would have expected use strict to generate a "use of uninitialized variable" error or something, but instead, it is a syntax error.

Can anyone please provide some enlightenment?

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Re: Complicated Hash Construction. by pg (Canon) on Dec 29, 2002 at 04:33 UTC
You can declare an entity, but not a member of an entity. This is what the syntax error is about. In your bad case, you tried to declare my @hash{@keyarray}, which is an element of an entity. It is NOT about the fact that you combined a declaration with an assignment. This is fine: `my $a = 1;` [download] and this is not fine: `my $a[1];` [download] This is also no good: `my @a; # this is okay my $a[1];# no good` [download]	[reply] [d/l] [select]
Re: Complicated Hash Construction. by MarkM (Curate) on Dec 29, 2002 at 08:14 UTC
The code: `my @hash{@keyarray} = @valuearray;` Should be understood as: `my($hash{$keyarray[0]}, $hash{$keyarray[1]}, ...) = @valuearray;` Once this is understood, it should be easier to see why Perl considers the expression to be a syntax error. Perl does not know what `my($hash{$key});` means. The exact error is `Can't declare hash slice in "my" at ...' For an interesting comparison, consider the following similar-looking code: `local @hash{@keyarray} = @valuearray;` This code actually works, because `local($hash{$key});` is perfectly valid and means 'override the value of $hash{$key} until this block completes'.	[reply] [d/l] [select]
Re: Complicated Hash Construction. by Trimbach (Curate) on Dec 29, 2002 at 04:26 UTC
I always thought it was because in the first example perl knows that %hash is, well, a hash. In the second example perl has no way of knowing whether @hash is a hash or an array. It looks like an array (and the right-hand side is in list context) and the braces look like a syntax error. There's no problem when you predeclare %hash because perl knows that @hash{ } is a hash slice and not an array. Those more knowledgable about the Perl internals can comment on why this syntax can't be DWIM'd into the interpreter, but I suspect it will not be an issue in Perl 6. Gary Blackburn Trained Killer	[reply]