On Mon, Sep 8, 2014 at 12:10 PM, Jaime Fernández del Río wrote:
On Mon, Sep 8, 2014 at 7:41 AM, Sturla Molden wrote:
Stefan Otte <stefan.otte@gmail.com> wrote:

>     stack([[a, b], [c, d]])
>
> In my case `stack` replaced `hstack` and `vstack` almost completely.
>
> If you're interested in including it in numpy I created a pull request
> [1]. I'm looking forward to getting some feedback!

As far as I can see, it uses hstack and vstack. But that means a and b have
to have the same number of rows, c and d must have the same rumber of rows,
and hstack((a,b)) and hstack((c,d)) must have the same number of columns.

Thus it requires a regularity like this:

AAAABB
AAAABB
CCCDDD
CCCDDD
CCCDDD
CCCDDD

What if we just ignore this constraint, and only require the output to be
rectangular? Now we have a 'tetris game':

AAAABB
AAAABB
CCCCBB
CCCCBB
CCCCDD
CCCCDD

or

AAAABB
AAAABB
CCCCBB
CCCCBB
CCCCBB
CCCCBB

This should be 'stackable', yes? Or perhaps we need another stacking
function for this, say numpy.tetris?

And while we're at it, what about higher dimensions? should there be an
ndstack function too?

This is starting to look like the second time in a row Stefan tries to extend numpy with a simple convenience function, and he gets tricked into implementing some sophisticated algorithm...

Maybe the third time is a gem.

For his next PR I expect nothing less than an NP-complete problem. ;-)

How about cholesky or qr updating?   I could use one right now.

Josef

Jaime

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