http://qs321.pair.com?node_id=1077457


in reply to Create unique array --the hard way!

Perhaps this will be suitable (for your class exercise): 

#!/usr/bin/env perl -l

use strict;
use warnings;

my @initial = qw{q w e r t y q w e r t y};
print "Initial: @initial";

my $last = '';
my @unique = map { $last eq $_ ? () : ($last = $_) } sort @initial;
print "Unique:  @unique";

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Output:

Initial: q w e r t y q w e r t y
Unique:  e q r t w y

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Update (alternative solution): You could even skip the creation of the unsorted array (your @array_q3): 

#!/usr/bin/env perl -l

use strict;
use warnings;

my $last = '';
my @unique = map { $last eq $_ ? () : ($last = $_) } sort map { split 
+} <DATA>;
print "Unique:  @unique";

__DATA__
I am supposed to create a unique array
I am not supposed to use hashes

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Output:

Unique:  I a am array create hashes not supposed to unique use

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-- Ken

Replies are listed 'Best First'.
Re^2: Create unique array --the hard way!
by Anonymous Monk on Mar 07, 2014 at 21:39 UTC
    Many thanks to all of you guys!!
[reply]
      I wonder, could this be done using the splice function as well? But how would you use it?
[reply]
[d/l]

        Well, you could but I don't know why you'd want to.

        In the following script (using Benchmark) you'll see:

        • splice is relatively less efficient. I also suspect it might have future maintainers scratching their heads wondering why it was used ("splice @array, 0, 1" has identical functionality to "shift @array" — it's just slower and requires more keystrokes to code).
        • shift is more efficient than splice.
        • My original map is faster than both of those; it doesn't destroy the original array; and it's a lot less coding. 
        #!/usr/bin/env perl

use strict;
use warnings;

use Benchmark qw{cmpthese};

cmpthese -1 => {
    splice_unique => \&splice_unique,
    shift_unique  => \&shift_unique,
    map_unique    => \&map_unique,
};

sub splice_unique {
    my @sorted = sort qw{q w e r t y q w e r t y};
    my $last = '';
    my @unique;

    while (@sorted) {
        my $element = splice @sorted, 0, 1;

        if ($last ne $element) {
            $last = $element;
            push @unique, $element
        }
    }
}

sub shift_unique {
    my @sorted = sort qw{q w e r t y q w e r t y};
    my $last = '';
    my @unique;

    while (@sorted) {
        my $element = shift @sorted;

        if ($last ne $element) {
            $last = $element;
            push @unique, $element
        }
    }
}

sub map_unique {
    my @sorted = sort qw{q w e r t y q w e r t y};
    my $last = '';
    my @unique = map { $last eq $_ ? () : ($last = $_) } @sorted;
}

        [download]

        Representative result:

                          Rate splice_unique  shift_unique    map_unique
splice_unique  93699/s            --           -5%          -20%
shift_unique   98642/s            5%            --          -16%
map_unique    117028/s           25%           19%            --

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        -- Ken

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