[Numpy-discussion] Use-case for np.choose

David Goldsmith d.l.goldsmith@gmail....
Sun Nov 8 22:08:39 CST 2009


On Sun, Nov 8, 2009 at 7:40 PM, Anne Archibald <peridot.faceted@gmail.com>wrote:

> As Josef said, this is not correct. I think the key point of confusion is
> this:
>
> Do not pass choose two arrays.
>
> Pass it one array and a *list* of arrays. The fact that choices can be
> an array is a quirk we can't change, but you should think of the
> second argument as a list of arrays,


Fine, but as you say, one *can* pass choose an array as the second argument
and it doesn't raise an exception, so if someone is stupid/careless enough
to pass an array for `choices`, how is choose interpreting it as a list?  Is
the first dimension "list converted" (so that, e.g., my (2,1,2) example is
interpreted as a two element list, each of whose elements is a (1,2) array)?


> possibly of different shapes.
>
> np.choose(np.ones((2,1,1)), [ np.ones((1,3,1)), np.ones((1,1,5)) ] )
>

>>> np.choose(np.ones((2,1,1)), [ np.ones((1,3,1)), np.ones((1,1,5)) ] )
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Python254\lib\site-packages\numpy\core\fromnumeric.py", line 208,
in
choose
    return choose(choices, out=out, mode=mode)
TypeError: array cannot be safely cast to required type

Here the first argument is an array of choices. The second argument is
> a *list* - if you cast it to an array you'll get an error or nonsense
>

So the result of my example was nonsense?


> - of arrays to choose from. The broadcasting ensures the first
> argument and *each element of the list* are the same shape. The only
> constraint on the number of arrays in the list is that it be larger
> than the largest value in a.
>
> If you try to make the second argument into a single array, for one
> thing you are throwing away useful generality by forcing each choice
> to be the same shape (and real shape, not zero-strided fake shape),
> and for another the broadcasting becomes very hard to understand.
>

Obviously, I find the broadcasting very hard to understand regardless. :-(

DG


>
> Anne
>
>
> 2009/11/8 David Goldsmith <d.l.goldsmith@gmail.com>:
> > OK, now I'm trying to wrap my brain around broadcasting in choose when
> both
> > `a` *and* `choices` need to be (non-trivially) broadcast in order to
> arrive
> > at a common shape, e.g.:
> >
> >>>> c=np.arange(4).reshape((2,1,2)) # shape is (2,1,2)
> >>>> a=np.eye(2, dtype=int) # shape is (2,2)
> >>>> np.choose(a,c)
> > array([[2, 1],
> >        [0, 3]])
> >
> > (Unfortunately, the implementation is in C, so I can't easily insert
> print
> > statements to see intermediate results.)
> >
> > First, let me confirm that the above is indeed an example of what I think
> it
> > is, i.e., both `a` and `choices` are broadcast in order for this to work,
> > correct?  (And if incorrect, how is one broadcast to the shape of the
> > other?)  Second, both are broadcast to shape (2,2,2), correct?  But how,
> > precisely, i.e., does c become
> >
> > [[[0, 1], [2, 3]],        [[[0, 1], [0, 1]],
> >  [[0, 1], [2, 3]]]   or   [[2, 3], [2, 3]]]
> >
> > and same question for a?  Then, once a is broadcast to a (2,2,2) shape,
> > precisely how does it "pick and choose" from c to create a (2,2) result?
> > For example, suppose a is broadcast to:
> >
> > [[[1, 0], [0, 1]],
> >  [[1, 0], [0, 1]]]
> >
> > (as indicated above, I'm uncertain at this point if this is indeed what a
> is
> > broadcast to); how does this create the (2,2) result obtained above?
> > (Obviously this depends in part on precisely how c is broadcast, I do
> > recognize that much.)
> >
> > Finally, a seemingly relevant comment in the C source is:
> >
> > /* Broadcast all arrays to each other, index array at the end.*/
> >
> > This would appear to confirm that "co-broadcasting" is performed if
> > necessary, but what does the "index array at the end" phrase mean?
> >
> > Thanks for your continued patience and tutelage.
> >
> > DG
> >
> > On Sun, Nov 8, 2009 at 5:36 AM, <josef.pktd@gmail.com> wrote:
> >>
> >> On Sun, Nov 8, 2009 at 5:00 AM, David Goldsmith <
> d.l.goldsmith@gmail.com>
> >> wrote:
> >> > On Sun, Nov 8, 2009 at 12:57 AM, Anne Archibald
> >> > <peridot.faceted@gmail.com>
> >> > wrote:
> >> >>
> >> >> 2009/11/8 David Goldsmith <d.l.goldsmith@gmail.com>:
> >> >> > On Sat, Nov 7, 2009 at 11:59 PM, Anne Archibald
> >> >> > <peridot.faceted@gmail.com>
> >> >> > wrote:
> >> >> >>
> >> >> >> 2009/11/7 David Goldsmith <d.l.goldsmith@gmail.com>:
> >> >> >> > So in essence, at least as it presently functions, the shape of
> >> >> >> > 'a'
> >> >> >> > *defines* what the individual choices are within 'choices`, and
> if
> >> >> >> > 'choices'
> >> >> >> > can't be parsed into an integer number of such individual
> choices,
> >> >> >> > that's
> >> >> >> > when an exception is raised?
> >> >> >>
> >> >> >> Um, I don't think so.
> >> >> >>
> >> >> >> Think of it this way: you provide np.choose with a selector array,
> >> >> >> a,
> >> >> >> and a list (not array!) [c0, c1, ..., cM] of choices. You
> construct
> >> >> >> an
> >> >> >> output array, say r, the same shape as a (no matter how many
> >> >> >> dimensions it has).
> >> >> >
> >> >> > Except that I haven't yet seen a working example with 'a' greater
> >> >> > than
> >> >> > 1-D,
> >> >> > Josef's last two examples notwithstanding; or is that what you're
> >> >> > saying
> >> >> > is
> >> >> > the bug.
> >> >>
> >> >> There's nothing magic about A being one-dimensional.
> >> >>
> >> >> C = np.random.randn(2,3,5)
> >> >> A = (C>-1).astype(int) + (C>0).astype(int) + (C>1).astype(int)
> >> >>
> >> >> R = np.choose(A, (-1, -C, C, 1))
> >> >
> >> > OK, now I get it: np.choose(A[0,:,:], (-1,-C,C,-1)) and
> >> > np.choose(A[0,:,0].reshape((3,1)), (-1,-C,C,1)), e.g., also work, but
> >> > np.choose(A[0,:,0], (-1,-C,C,-1)) doesn't - what's necessary for
> >> > choose's
> >> > arguments is that both can be broadcast to a common shape (as you
> state
> >> > below), but choose won't reshape the arguments for you to make this
> >> > possible, you have to do so yourself first, if necessary.  That does
> >> > appear
> >> > to be what's happening now; but do we want choose to be smarter than
> >> > that
> >> > (e.g., for np.choose(A[0,:,0], (-1,-C,C,-1)) to work, so that the user
> >> > doesn't need to include the .reshape((3,1)))?
> >>
> >> No, I don't think we want to be that smart.
> >>
> >> If standard broadcasting rules apply, as I think they do, then I
> wouldn't
> >> want
> >> any special newaxis or reshapes done automatically. It will be
> confusing,
> >> the function wouldn't know what to do if there are, e.g., as many rows
> as
> >> columns, and this looks like a big source of errors.
> >> Standard broadcasting is pretty nice (once I got the hang of it), and
> >> adding
> >> a lot of np.newaxis (or some reshapes) to the code is only a small price
> >> to pay.
> >>
> >> Josef
> >>
> >>
> >>
> >> >
> >> > DG
> >> >
> >> >>
> >> >> Requv = np.minimum(np.abs(C),1)
> >> >>
> >> >> or:
> >> >>
> >> >> def wedge(*functions):
> >> >>     """Return a function whose value is the minimum of those of
> >> >> functions"""
> >> >>     def wedgef(X):
> >> >>          fXs = [f(X) for f in functions]
> >> >>          A = np.argmin(fXs, axis=0)
> >> >>          return np.choose(A,fXs)
> >> >>     return wedgef
> >> >>
> >> >> so e.g. np.abs is -wedge(lambda X: X, lambda X: -X)
> >> >>
> >> >> This works no matter what shape of X the user supplies - so a wedged
> >> >> function can be somewhat ufunclike - by making A the same shape.
> >> >>
> >> >> >> The (i0, i1, ..., iN) element of the output array
> >> >> >> is obtained by looking at the (i0, i1, ..., iN) element of a,
> which
> >> >> >> should be an integer no larger than M; say j. Then r[i0, i1, ...,
> >> >> >> iN]
> >> >> >> = cj[i0, i1, ..., iN]. That is, each element of the selector array
> >> >> >> determines which of the choice arrays to pull the corresponding
> >> >> >> element from.
> >> >> >
> >> >> > That's pretty clear (thanks for doing my work for me). ;-), Yet,
> see
> >> >> > above.
> >> >> >
> >> >> >> For example, suppose that you are processing an array C, and have
> >> >> >> constructed a selector array A the same shape as C in which a
> value
> >> >> >> is
> >> >> >> 0, 1, or 2 depending on whether the C value is too small, okay, or
> >> >> >> too
> >> >> >> big respectively. Then you might do something like:
> >> >> >>
> >> >> >> C = np.choose(A, [-inf, C, inf])
> >> >> >>
> >> >> >> This is something you might want to do no matter what shape A and
> C
> >> >> >> have. It's important not to require that the choices be an array
> of
> >> >> >> choices, because they often have quite different shapes (here, two
> >> >> >> are
> >> >> >> scalars) and it would be wasteful to broadcast them up to the same
> >> >> >> shape as C, just to stack them.
> >> >> >
> >> >> > OK, that's a pretty generic use-case, thanks; let me see if I
> >> >> > understand
> >> >> > it
> >> >> > correctly: A is some how created independently with a 0 everywhere
> C
> >> >> > is
> >> >> > too
> >> >> > small, a 1 everywhere C is OK, and a 2 everywhere C is too big;
> then
> >> >> > np.choose(A, [-inf, C, inf]) creates an array that is -inf
> everywhere
> >> >> > C
> >> >> > is
> >> >> > too small, inf everywhere C is too large, and C otherwise (and
> since
> >> >> > -inf
> >> >> > and inf are scalars, this implies broadcasting of these is taking
> >> >> > place).
> >> >> > This is what you're asserting *should* be the behavior.  So, unless
> >> >> > there is
> >> >> > disagreement about this (you yourself said the opposite viewpoint
> >> >> > might
> >> >> > rationally be held) np.choose definitely presently has a bug,
> namely,
> >> >> > the
> >> >> > index array can't be of arbitrary shape.
> >> >>
> >> >> There seems to be some disagreement between versions, but both Josef
> >> >> and I find that the index array *can* be arbitrary shape. In numpy
> >> >> 1.2.1 I find that all the choose items must be the same shape as it,
> >> >> which I think is a bug.
> >> >>
> >> >> What I suggested might be okay was if the index array was not
> >> >> broadcasted, so that the outputs always had exactly the same shape as
> >> >> the index array. But upon reflection it's useful to be able to use a
> >> >> 1-d array to select rows from a set of matrices, so I now think that
> >> >> all of A and the elements of choose should be broadcast to the same
> >> >> shape. This seems to be what Josef observes in his version of numpy,
> >> >> so maybe there's nothing to do.
> >> >>
> >> >> Anne
> >> >>
> >> >> > DG
> >> >> >
> >> >> >>
> >> >> >> Anne
> >> >> >> _______________________________________________
> >> >> >> NumPy-Discussion mailing list
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> >> >> >> http://mail.scipy.org/mailman/listinfo/numpy-discussion
> >> >> >
> >> >> >
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> >> >> >
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