why does sort with uniq not work together

[david2008]

Hi, This is a crosslist from stack overflow The problem is the following

use strict;
use List::MoreUtils qw/uniq/;
use Data::Dumper;

my @x = (3,2);
my @y = (4,3);

print "unique results \n";
print Dumper([uniq(@x,@y)]);

print "sorted unique results\n";
print Dumper([sort uniq(@x,@y)]);

gives as output

unique results 
$VAR1 = [
          3,
          2,
          4
        ];
sorted unique results
$VAR1 = [
          2,
          3,
          3,
          4
        ];

Unfortunately the uniq does not work anymore.
There is an answer from amon that perl uses the sort function form where the first parameter is a function and the second a list, but i still don't understand why it does not execute uniq beforehand and then uses the sort LIST,instead of preferring the sort SUBNAME LIST form.

Amon also suggested to use
sort +uniq @letters;
What does the + mean?
Thanks,
David

[tobyink]

The + is the unary plus operator. It does nothing, but can be used to disambiguate a first argument. For example:

print (1 + 2) . "\n";

You might expect that to be roughly the same as print "3\n", but it's actually more like my $tmp = print 3; $tmp . "\n". The line break doesn't get printed; it gets concatenated to the return value of the print function. That is, it's parsed like:

(print(1+1)) . "\n";

The unary plus helps Perl notice that something is an argument to the preceding function:

print +(1 + 2) . "\n";

Unary plus can also be useful in disambiguating +{} (a hashref) from { } (an empty block of code).

See Symbolic Unary Operators in perlop.

[2teez]

Hi david2008,
If you use warnings; you would have seen this message: Sort subroutine didn't return single value at line ...
so using this

print "sorted unique results\n";
print Dumper( [ sort( uniq( @x, @y ) ) ] );

would resolve the issue. Try it

[hdb]

Yes, but why?

[2teez]

Hi hdb,
Yes, but why?
Checking sort documentation you will find this:

Warning: syntactical care is required when sorting the list returned from a function. If you want to sort the list returned by the function call "find_records(@key)", you can use:

                   @contact = sort { $a cmp $b } find_records @key;
                   @contact = sort +find_records(@key);
                   @contact = sort &find_records(@key);
                   @contact = sort(find_records(@key));

Among other things..
That shows it can as well use: print Dumper( [ sort &uniq( @x, @y )  ] ); showing the list to be sorted is returned from a function.

[choroba]

why it does not execute uniq beforehand

But it does, it executes uniq as the function that tells sort how to compare elements, as in

sort { uniq($a, $b) } @x, @y;

[Anonymous Monk]

Looks like it :)

$ perl -le " sub uniq {warn++$q;@_} print for sort uniq(1,8,4,3) "
1 at -e line 1.
2 at -e line 1.
3 at -e line 1.
4 at -e line 1.
1
8
4
3

$ perl -MO=Deparse -le " sub uniq {warn++$q;@_} print for sort uniq(1,
+8,4,3) "
BEGIN { $/ = "\n"; $\ = "\n"; }
sub uniq {
    warn ++$q;
    @_;
}
print $_ foreach (sort uniq 1, 8, 4, 3);
-e syntax OK