While using map certainly works, using map in a void context like you do is inefficient. Map returns an array for you regardless of whether you use it or not. As a result using map as a surrogate for loop is bad practice. In golf you use it because it is 2 chars shorter than a for loop. That is about the only (valid) reason to use it in a void context :-) All you actually want is:

$help{$_}++ for @a
[download]

This is far more efficient, it is also shorter for what it's worth. Thanks to chipmunk for straigtening me out on this some months ago.

my @a= (1,2,3,4,5,6,6,5,4,3,2,1);
my %help;
my @wasted_array = map { $help{$_}=1 } @a;
@a = keys %help;
print @wasted_array, "\n", @a;
[download]

cheers

tachyon

s&&rsenoyhcatreve&&&s&n.+t&"$'$`$\"$\&"&ee&&y&srve&&d&&print

If you are looking an efficient method to do this then try.

my %hash;
@hash{@a} = ();
@a = keys %hash;

But that only works, just as your example, for non-references as the keys of a hash can only be strings. If @a may contain references that you want to preserve then use

my %hash;
@hash{@a} = @a;
@a = values %hash;

This is slightly less efficient as it causes an extra copy of each element in @a

I generally write this as

@a = do { my %h; @h{@a} = @a; values %h }; # unique

As you say, the slice method is faster again. I will be using it from now on. Thanks.

use Benchmark;

@a= (0..100,0..100);

$mine = <<'CODE';
my %hash;
@hash{$_}++ for @a;
@a = keys %hash;
CODE

$yours1 = <<'CODE';
my %hash;
@hash{@a} = ();
@a = keys %hash;
CODE

$yours2 = <<'CODE';
my %hash;
@hash{@a} = @a;
@a = values %hash;
CODE

$his = <<'CODE';
my %hash;
map { $hash{$_}=1 } @a;
@a = keys %hash;
CODE

timethese ( 10000, { 'mine'   => $mine, 
                     'yours1' => $yours1,
                     'yours2' => $yours2, 
                     'his'    => $his 
                   } );

__END__
C:\>perl test.pl
Benchmark: timing 10000 iterations of his, mine, yours1, yours2...
       his: 17 wallclock secs (17.03 usr +  0.00 sys = 17.03 CPU) @ 58
+7.20/s (n=10000)
      mine: 15 wallclock secs (15.05 usr +  0.00 sys = 15.05 CPU) @ 66
+4.45/s (n=10000)
    yours1: 12 wallclock secs (11.76 usr +  0.00 sys = 11.76 CPU) @ 85
+0.34/s (n=10000)
    yours2: 15 wallclock secs (14.78 usr +  0.00 sys = 14.78 CPU) @ 67
+6.59/s (n=10000)

C:\>
[download]

cheers

tachyon

s&&rsenoyhcatreve&&&s&n.+t&"$'$`$\"$\&"&ee&&y&srve&&d&&print

Hi thank u so much for your code ..it is working it has solved my problem ..but as a beginner i want know wats happening there ..could plz explain me the code..thanks in advance.