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    <p>I had a look at the very first piece of code on the page you referred to:</p> <pre><code>sub c{my$n=-1+shift;$n?map{my$c=$_;map$c.$_,c($n,@_)}@_:@_} </code></pre> <p>I have spread it out a bit to make it more readable; also I have made some changes to it to make it clearer (see <code>combinations</code>):</p> <pre><code>#!/usr/bin/perl use strict; use warnings; sub c { my $n=-1+shift; $n ? map{ my $c = $_; map $c . $_ , c($n ,@_) } @_ : @_; } sub combinations { my $number = shift; # remove the first item from @_ my @chars = @_; # the remainder of @_ $number --; # decrement $number, so that you will eventually exit # from this recursive subroutine (once $number == 0) if ($number) { # true as long as $number != 0 and $number not undef my @result; foreach my $char (@chars) { my @intermediate_list = map { $char . $_ } combinations($number, @chars); push @result, @intermediate_list; } return @result; # the current concatenation result will be used for creation of # @intermediate_list in the 'subroutine instance' that called 'combinations' } else { return @chars; } } print join " ", combinations(2, "A", "B"); print "\n"; print join " ", c(2, "A", "B"); print "\n\n"; print join " ", combinations(3, "A", "B"); print "\n"; print join " ", c(3, "A", "B"); print "\n"; </code></pre> <p>Both versions work in the same way, and they produce exactly the same output:</p> <pre><code>AA AB BA BB AA AB BA BB AAA AAB ABA ABB BAA BAB BBA BBB AAA AAB ABA ABB BAA BAB BBA BBB </code></pre> <p>I included some comments in the code, but perhaps a lengthier explanation is in order!? Well, here's an example to illustrate how things work: let's say we've got two items, "A" and "B", and we want to get all possible combinations of 2 of these items. In that case, <code>$number</code> will initially be equal to 2 (as we want to get pairs), and <code>@chars</code> will be equal to <code>('A', 'B')</code>.</p> <p>The first time <code>combinations</code> is called, <code>$number</code> is decremented to 1, thus the <code>if</code> condition is met, and we enter the <code>foreach</code> loop. This first sets <code>$char</code> to 'A'. It then calls <code>combinations(1, ('A', 'B'))</code>. As <code>$number</code> always gets decremented when the subroutine is called, <code>$number</code> is 0 in this 'child subroutine', consequently the child simply returns ('A', 'B'). Thus:</p> <pre><code>@intermediate_list = map { $char . $_ } ('A', 'B'); # $char eq 'A' </code></pre> <p><code>map</code> then takes both 'A' and 'B' and concatenates each with 'A' ($char), thus <code>@intermediate_list</code> is ('AA', 'AB'). In the next round of the <code>foreach</code> loop, the same is done with <code>$char = B</code>, which sets <code>@intermediate_list</code> to ('BA', 'BB').</p> <p>In each round the contents of <code>@intermediate_list</code> are pushed into the result list, hence <code>@result</code> eventually contains all possible combinations.</p> <p>If you want to get triplets instead of pairs, you will obviously start with <code>$number = 3</code>, and <code>combinations</code> will be called three times. The second time it's called it will return <code>@result</code>, i.e. a pair-containing list. Each item from that list will be concatenated with each character of the initial character set.</p> <p>Okay, I hope this makes sense. Please ask in case something has not become clear.</p> <p><strong>EDIT</strong>: Please see ysth's comment below.</p>
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    1. POHow can I generate all ordered combinations of length k in Perl?
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    1. CO"will be called three times" should be "will recurse to a depth of 3"; there are in fact many more than three calls (unless you just have one letter...)
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    2. CO@ysth Okay, that makes sense. I have never worked with recursive subroutines before, therefore I am not familiar with the terms. Thanks for pointing out the error!
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