Hey, nice one, Derek....
Er, you can also use the threading engine:
perldl> ($n,$m) = (3,2);
perldl> ($a,$b) = map { sequence($_) } ($n,$m);
perldl> $c = cat($a->(:,*2), $b->(*3,:))->clump(2)->mv(-1,0);
perldl> print " a=$a \n b=$b \n c=$c \n";
a=[0 1 2]
b=[0 1]
c=
[
[0 0]
[1 0]
[2 0]
[0 1]
[1 1]
[2 1]
]
This works by padding each array with a dummy dimension: $a->(:,*3) is
a 2x3-PDL, as is $b->(*2,:). Catting them together gives a 2x3x2-PDL,
whose 0th dim runs across elements of $a, whose 1st dim runs across
elements of $b, and whose 2nd dim runs between the $a and $b pdls.
Clumping the first two dimensions gives a 6x2 PDL whose 0th dim runs
across index and whose 1st dim runs between $a and $b. The final mv
changes it to a 2x6 pdl, which is what you asked for.
This sort of operation (padding dummied variables) is related to the
outer product. In fact, outer product can be implemented as:
sub outer {
my ($a,$b)=...@_;
return $a->(:,*($b->dim(0))) * $b->(*($a->dim(0)),:);
}
The difference is that, instead of multiplication, we're doing
assembly of vectors.
Best,
Craig
print $a->(*1)
On Feb 17, 2009, at 2:25 PM, Derek Lamb wrote:
> InSuk Joung wrote:
>> Hi Users,
>> I am wondering if there is a function that make a permutation piddle.
>>
>> For example,
>>
>> $a = sequence(2);
>> $b = sequence(3);
>>
>> Using the two piddles above, I want to create
>>
>> [
>> [ 0, 0 ],
>> [ 0, 1 ],
>> [ 0, 2 ],
>> [ 1, 0 ],
>> [ 1, 1 ],
>> [ 1, 2 ],
>> ]
>>
>> Any suggestion would be appreciated!
>>
> It's OK to use a for-loop sometimes! That's probably the easiest.
> But
> since you asked:
>
> $n = $a->nelem * $b->nelem;
> use PDL::Transform;
> $ap = $a->dummy(0)->map(t_identity,[1,$n],{method=>'s'});
> $bp = $b->range(0,$n,'p')->dummy(0) ;
> $p = append($ap,$bp);
>
> cheers,
> Derek
>
>
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