Python, Perl: Generate All Possible Pairings

By Xah Lee. Date:

Given a list that is a set partitioned into subsets, generate a list of all possible pairings of elements in any two subset.

Example:

 genpair( [[9,1],[5],[2,8,7]] );

returns:

 [[9,5],[1,5],[9,2],[9,8],[9,7],[1,2],[1,8],[1,7],[5,2],[5,8],[5,7]]

(we do not assume the given set has order, so the result is not
ordered.)

Perl code and Python code will be posted in 2 days. Answer below.

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Perl

# -*- coding: utf-8 -*-
# perl

sub genpair ($) {
my $partiSet = $_[0];
my @result;
for (my $head =0; $head <= ((scalar @$partiSet)-2); $head++ ) {
    for (my $tail = $head+1; $tail <= ((scalar @$partiSet)-1); $tail++ ) {
        foreach my $ii (@{$partiSet->[$head]}) {
            foreach my $jj (@{$partiSet->[$tail]}) {
                push @result, [$ii,$jj]
            }
        }
    }
}
return \@result;
}

Python

# -*- coding: utf-8 -*-
# python

def genpair (partiSet):
    result=[]
    for head in range(len(partiSet)-1):
        for tail in range(head+1,len(partiSet)):
            for ii in partiSet[head]:
                for jj in partiSet[tail]:
                    result.append((ii,jj))
    return result

# by Sean Gugler
# same algorithm as above, different syntax
def genpair2 (partiSet):
    return [(a,b) for s in range(len(partiSet))
            for t in range(s)
            for a in partiSet[t]
            for b in partiSet[s]]

ll=( [[9,1],[5],[5],[2,8,7],[2]])
t=genpair(ll)
t2=genpair2(ll)
if (t.sort()==t2.sort()): print 'yay'

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