keys with an expression

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

this is a way to get the unique elements of an array:

my @array = (1,1,2,3,2,2);
my %temp = map { $_ => 1 } @array;
my @unique = keys %temp;
[download]

I would like to do away with the temporary hash and would like to write the following seemingly natural perl-code:

my @unique = keys (map { $_ => 1 } @array);
[download]

But to my amazement this does not work with 5.18.1 - the error is "Type of argument to keys on reference must be unblessed hashref or arrayref".

I could swear this used to work some years ago (not really sure), so my question is how can I do this - how can I use keys with such an expression?

Is there really no way to avoid the ugly temporary?

Please note that the above is just an illustration - this is not a question about how to find unique array elements but a question concerning the kinds of expressions I can feed into keys.

Comment on keys with an expression Select or Download Code

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Re: keys with an expression by hdb (Monsignor) on Nov 20, 2013 at 09:49 UTC
This should work in the newer Perls: `my @unique = keys { map { $_ => 1 } @array };` [download]	[reply] [d/l]
Re: keys with an expression (List::AllUtils::uniq by Anonymous Monk on Nov 20, 2013 at 10:08 UTC
create a hash,ref { } dereference same hash,ref %{ { } } keys likes it `use Data::Dump; dd( keys %{{ map { $_,1 } 1,2,1,2,1,4 }} ); __END__ (4, 1, 2)` [download] List::AllUtils :)	[reply] [d/l]
Re: keys with an expression by tobyink (Canon) on Nov 20, 2013 at 12:57 UTC
This does create a temporary hash, but avoids the need to choose a name for the hash: `my @items = qw( foo bar bar baz quux baz ); my @unique = local(*_) && grep(!$_{$_}++, @items); print "$_\n" for @unique;` [download] `use Moops; class Cow :rw { has name => (default => 'Ermintrude') }; say Cow->new->name`	[reply] [d/l]
Re: keys with an expression by lune (Pilgrim) on Nov 20, 2013 at 15:04 UTC
I don't know if that worked before, but I do find the error message okay, as `(map { $_ => 1 } @array)` [download] returns a list, not an array, which "keys" would also accept as a valid argument. You get the same error if you try: `my @unique = keys (1,1,2,3,2,2);` [download] but not with: `my @unique = keys @array;` [download] which returns the indices of the array.	[reply] [d/l] [select]
Re: keys with an expression by AnomalousMonk (Archbishop) on Nov 20, 2013 at 19:16 UTC
... this is ... a question concerning the kinds of expressions I can feed into keys. Answers to questions like this can be had by Reading about keys and other, er, keywords (and much else besides) in The Fine Manual(s). (And BTW: Feeding `keys` et al a hash/array reference works from 5.14 on.)	[reply] [d/l]


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