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### generate combinations of lists

by gpm1982 (Initiate)
 on Sep 28, 2005 at 05:04 UTC Need Help??
gpm1982 has asked for the wisdom of the Perl Monks concerning the following question:

hello there.
i need some help here. what im trying to do is to list out combinations of a given array of strings, numbers, etc.

for example:
if i had an array like qw(a b c), i would like to generate combinations of the array above, so that i would have:
qw(a b c), qw(a c b), qw(b a c), qw(b c a), qw(c a b), and qw(c b a)

ps: to find out how many different combinations can be produced from an array of n elements, use factorial (!). for example, if i had an array of 5 elements, my combinations will have a total of
5! = 5x4x3x2x1 = 120 different combinations

Replies are listed 'Best First'.
Re: generate combinations of lists
by ikegami (Pope) on Sep 28, 2005 at 06:07 UTC

Those are permutations, not combinations. Order doesn't matter for combinations.

Algorithm::Loops has a function called NextPermute which iterates permutations. Taken almost verbatim from the documentation:

use Algorithm::Loops qw( NextPermute ); my @list = sort qw( a b c ); do { print(join(', ', @list), "\n"); } while (NextPermute(@list));
Re: generate combinations of lists
by gargle (Hermit) on Sep 28, 2005 at 07:27 UTC

Hi,

update: clarification

disclaimer: bogoperm, but it does fit your description of the problem!

#!/usr/bin/perl use strict; use warnings; ############# # bogoperm! # ############# my @array = qw(a b c); my \$length = @array; # faculty my \$fac = 1; foreach my \$f (1..\$length) { \$fac *= \$f; } # my results my %hash = (); # internals my \$check; # if equals control then we found a number my \$control; # has to contain something like 123... my \$index; # the number itself, it's an index! my \$count = 0; # 'till this equals \$fac OUTER: do { INNER: while (1) { do { \$check = 0; \$control = 0; \$index = 0; foreach my \$i (1..\$length) { my \$number = 1+int(rand()*\$length); \$index += \$number * 10**(\$i-1); \$check += 10**\$number; \$control += 10**\$i; } \$check /= 10; \$control /= 10; } while (\$check != \$control); last INNER if defined \$hash{\$index}; \$hash{\$index}++; \$count++; } } while \$count != \$fac; # print out the results foreach my \$i (keys %hash) { my \$j; do { \$j = \$i % 10; print \$array[\$j-1]; \$i = (\$i - \$j) / 10; } while (\$i > 0); print "\n"; }

output:

bash-3.00\$ ./perm.pl cba acb bac cab abc bca

I'll leave it as an exercise to put the output in real lists.

Well, at least I used strict and warnings :o

--
if ( 1 ) { \$postman->ring() for (1..2); }
No ++ from me because you work with "rand". That's not permutating but "throwing dice" and hoping to get all permutations.

-- for doing someone's homework ;-)

\$\=~s;s*.*;q^|D9JYJ^^qq^\//\\\///^;ex;print
No ++ from me because you work with "rand". That's not permutating but "throwing dice" and hoping to get all permutations.

Um, I think that's the point of it — it's a joke! (Especially considering its name.)

-- for doing someone's homework ;-)

But surely approaching a homework question in an intentionally stupid way that they wouldn't get any marks doesn't actually count as doing somebody's homework? I thought gargle's node was humourous, and can't see what's so harmful about it to warrant its negative reputation.

Smylers

++ for correcting me ;-)

Nevertheless: Still no ++ for the answer because rand is not something I would consider a good solution for permutation. So it's not worth a good reputation. Neither is mine so the (currently) -1 is okay

\$\=~s;s*.*;q^|D9JYJ^^qq^\//\\\///^;ex;print
Re: generate combinations of lists
by planetscape (Chancellor) on Sep 28, 2005 at 05:13 UTC

Please see this. Show us what you have already tried in order to solve the problem yourself.

HTH,

planetscape
Re: generate combinations of lists
by kprasanna_79 (Hermit) on Sep 28, 2005 at 07:16 UTC
Hi gpm,

-Prasanna.K
Re: generate combinations of lists
by Skeeve (Vicar) on Sep 28, 2005 at 07:19 UTC
Sounds like homework and I don't think the second answer is the one the teacher wants to see ;-)

\$\=~s;s*.*;q^|D9JYJ^^qq^\//\\\///^;ex;print

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