If PDL-like threading syntax is adopted, this is trivial. In PDL:
$a = pdl(1,2);
$b = pdl(3,4);
$c = $a->(*1) * $b;
print $c;
yields the output:
[
[3 4]
[6 8]
]
The '(*1)' inserts a dummy dimension into $a, making it a 1x2-array
rather than a 2-array. Then
the threading engine makes the appropriate outer product.
I strongly encourage the use of threading syntax (see the very nice
PDL::NiceSlice module) rather than a specific outer-product operator:
threading syntax turns out to be the answer to a rather large
collection of problems.
On Oct 28, 2005, at 12:21 AM, Darren Duncan wrote:
Not sure if this matter was resolved on a previous discussion, but
here goes ...
I would like to have a simple way to combine 2 array where every
element of each array is combined with every element of the other
array; this can also chain or scale to handle any number of arrays.
For now lets name it 'cross', since it behaves similarly to the set
operation that SQL calls a cross-join.
What I'm not yet sure about is whether this would be better as
something resembling the zip() operator or a hyper-operator.
One thing I would like to be able to do is this:
@baz = cross([1,2],[3,4]); # yields ([1,3],[1,4],[2,3],[2,4])
And alternately, this:
for cross([1,2],[3,4]) -> $foo,$bar { ... } # loop has 4 iterations
More examples:
cross() # yields ()
cross([],[1]) # yields ()
cross([1,2]) # yields ([1],[2])
cross([1,2],[3]); # yields ([1,3],[2,3])
cross([1],[2],[3]) # yields ([1,2,3])
cross([1],[2],[3,4]) # yields ([1,2,3],[1,2,4])
The order of the output elements is determined by the order of the
input elements.
If one were to be able to use this as a simple joining mechanism,
then each combination would have to be returned as an array, so
that 'map' or 'grep' etc would work properly. For example:
@A = ('red shirt', 'white shirt');
@B = ('blue pants', 'black pants');
@C = map { "going with $_[0] and $_[1] today" } cross(@A,@B);
On the other hand, perhaps something I actually want is something
like the hyper-operation but with appropriately different syntax:
['a','b'] >>~<< ['c','d']
But that it returns ['ac','ad','bc','bd'] rather than ['ac','bd'].
So is there a similarly terse way to do combinations already, and
if not then would you consider it commonly used enough to add?
-- Darren Duncan
