[Numpy-discussion] using scalar input on np.PyArray_MultiIterNew2

[zoj613]

Hi all,

I am looking for a way to use `np.PyArray_MultiIterNew2` in Cython to
broadcast parameters of a function. The requirement is that the two
arguments can be scalar and/or sequences. Using the usual `np.broadcast`
function works well but is slow when iterating over the broadcasted input in
a tight loop. I want to achieve the same using the C API.

Currently, if I used `(np.PyArray_MultiIter_DATA(bcast, i))[0]` to
iterate over the input when one of them is a scalar,
I get no errors, but I notice the output of the parent function returns an
array of zeros, which implies this approach didn't work. After
investigating, it seems that np.PyArray_MultiIter_DATA only accepts numpy
arrays.

I could write a function to handle all combinations of
scalar/array/list/tuple, and create temporary arrays to store the input
data, but that seems daunting and error prone. Is there a way I can achieve
this and have scalar arguments passed to np.PyArray_MultiIter_DATA be
converted to same-element arrays without writing my own code from scratch?

Any suggestions are welcome.



--
Sent from: http://numpy-discussion.10968.n7.nabble.com/
___
NumPy-Discussion mailing list
NumPy-Discussion@python.org
https://mail.python.org/mailman/listinfo/numpy-discussion

Re: [Numpy-discussion] using scalar input on np.PyArray_MultiIterNew2

[Sebastian Berg]

On Sun, 2021-01-10 at 09:59 -0700, zoj613 wrote:
> Hi all,
> 
> I am looking for a way to use `np.PyArray_MultiIterNew2` in Cython to
> broadcast parameters of a function. The requirement is that the two
> arguments can be scalar and/or sequences. Using the usual
> `np.broadcast`
> function works well but is slow when iterating over the broadcasted
> input in
> a tight loop. I want to achieve the same using the C API.

NB: There is also a different, newer, iterator (`NpyIter_New`).
Depending on what you do, that might be a lot faster and better
(although, I admit it tends to be more verbose too).
That iterator also does broadcasting, and one nice property is that it
can allocate a result array.  Most importantly, it casts for you and
allows you to take charge of the inner-loop (a one dimensional loop)
for performance.  (This is the core of how ufuncs work.)

> Currently, if I used `(np.PyArray_MultiIter_DATA(bcast,
> i))[0]` to
> iterate over the input when one of them is a scalar,
> I get no errors, but I notice the output of the parent function
> returns an
> array of zeros, which implies this approach didn't work. After
> investigating, it seems that np.PyArray_MultiIter_DATA only accepts
> numpy
> arrays.

I am honestly confused by this (did you mean a different command?).
`PyArray_MultiIter_DATA` returns a pointer to the raw data, i.e. in
cython you would probably do:

cdef double *value_ptr = npc.PyArray_MultiIter_DATA(iter, 0)
value_ptr[0] = 3.1416

Do want to reference the original data?  You could reference the
original data for scalars (read-only since scalars are immutable), but
for lists/tuples there is no original data to begin with (unless you
have Python objects inside, but it would seem strange if NumPy to
attempted to transparently expose that).

> I could write a function to handle all combinations of
> scalar/array/list/tuple, and create temporary arrays to store the
> input

The typical pattern is to convert everything to array first, but
`PyArray_MultiIter()` does that for you. Unless you want to optimize
that conversion away?

Cheers,

Sebastian


> data, but that seems daunting and error prone. Is there a way I can
> achieve
> this and have scalar arguments passed to np.PyArray_MultiIter_DATA be
> converted to same-element arrays without writing my own code from
> scratch?
> 
> Any suggestions are welcome.
> 
> 
> 
> --
> Sent from: http://numpy-discussion.10968.n7.nabble.com/
> ___
> NumPy-Discussion mailing list
> NumPy-Discussion@python.org
> https://mail.python.org/mailman/listinfo/numpy-discussion
> 



signature.asc
Description: This is a digitally signed message part
___
NumPy-Discussion mailing list
NumPy-Discussion@python.org
https://mail.python.org/mailman/listinfo/numpy-discussion

Re: [Numpy-discussion] using scalar input on np.PyArray_MultiIterNew2

[zoj613]

 
Sebastian Berg wrote
> On Sun, 2021-01-10 at 09:59 -0700, zoj613 wrote:
>> Hi all,
>> 
>> I am looking for a way to use `np.PyArray_MultiIterNew2` in Cython to
>> broadcast parameters of a function. The requirement is that the two
>> arguments can be scalar and/or sequences. Using the usual
>> `np.broadcast`
>> function works well but is slow when iterating over the broadcasted
>> input in
>> a tight loop. I want to achieve the same using the C API.
> 
> NB: There is also a different, newer, iterator (`NpyIter_New`).
> Depending on what you do, that might be a lot faster and better
> (although, I admit it tends to be more verbose too).
> That iterator also does broadcasting, and one nice property is that it
> can allocate a result array.  Most importantly, it casts for you and
> allows you to take charge of the inner-loop (a one dimensional loop)
> for performance.  (This is the core of how ufuncs work.)
> 
>> Currently, if I used `(np.PyArray_MultiIter_DATA(bcast,
>> i))[0]` to
>> iterate over the input when one of them is a scalar,
>> I get no errors, but I notice the output of the parent function
>> returns an
>> array of zeros, which implies this approach didn't work. After
>> investigating, it seems that np.PyArray_MultiIter_DATA only accepts
>> numpy
>> arrays.
> 
> I am honestly confused by this (did you mean a different command?).
> `PyArray_MultiIter_DATA` returns a pointer to the raw data, i.e. in
> cython you would probably do:
> 
> cdef double *value_ptr = 
> 
> npc.PyArray_MultiIter_DATA(iter, 0)
> value_ptr[0] = 3.1416
> 
> Do want to reference the original data?  You could reference the
> original data for scalars (read-only since scalars are immutable), but
> for lists/tuples there is no original data to begin with (unless you
> have Python objects inside, but it would seem strange if NumPy to
> attempted to transparently expose that).
> 
>> I could write a function to handle all combinations of
>> scalar/array/list/tuple, and create temporary arrays to store the
>> input
> 
> The typical pattern is to convert everything to array first, but
> `PyArray_MultiIter()` does that for you. Unless you want to optimize
> that conversion away?
> 
> Cheers,
> 
> Sebastian
> 
> 
>> data, but that seems daunting and error prone. Is there a way I can
>> achieve
>> this and have scalar arguments passed to np.PyArray_MultiIter_DATA be
>> converted to same-element arrays without writing my own code from
>> scratch?
>> 
>> Any suggestions are welcome.
>> 
>> 
>> 
>> --
>> Sent from: http://numpy-discussion.10968.n7.nabble.com/
>> ___
>> NumPy-Discussion mailing list
>> 

> NumPy-Discussion@

>> https://mail.python.org/mailman/listinfo/numpy-discussion
>> 
> 
> I think I wasn't clear enough in my original question. Here is what I
> have:
> a python function with the signature: def pyfunc(a, b). This function
> calls another cython function internally such that
> def pyfunc(a, b):
> 
> return cfunc(a, b)
> 
> I want the pyfunc to support scalar and array_like input for both
> parameters, so that internally I can have:
> def pyfunc(a, b):
> 
> # some part of the function
> for i, j in some_broadcasted_iterator:
> output[indx] = cfunc(i, j)
> 
> Broadcasting using python level functions allows me to do this but then
> the iteration part is slow because it uses python python code, meaning I
> can't release the gil. What I was thinking of doing is using the C API to
> create the broadcasted iterator using PyArray_MultiIterNew2 then iterating
> over that. I kept getting a zero array as output when I tested it so I
> assumed that the function doesnt work with scalars. It turns out the bug
> was caused by me using integer scalars when the C function expects doubles
> (But I fixed that after creating this post).
> 
> Now I'm left with figuring out how I can efficiently convert even
> lists/tuple inputs into a broadcasted iterator (without requiring the user
> to pass numpy arrays). When I pass a list for parameter a and a scalar for
> parameter b, the program hangs, so Im not sure if the
> PyArray_MultiIterNew2 function does it for me?
> 
> Btw, do you have a link to the more recent NpyIter_New? I couldn't find it
> reading the C-API page in the docs.
> 
> 
> 
> ___
> NumPy-Discussion mailing list

> NumPy-Discussion@

> https://mail.python.org/mailman/listinfo/numpy-discussion
> 
> 
> signature.asc (849 bytes)
> ;





--
Sent from: http://numpy-discussion.10968.n7.nabble.com/
___
NumPy-Discussion mailing list
NumPy-Discussion@python.org
https://mail.python.org/mailman/listinfo/numpy-discussion

Re: [Numpy-discussion] using scalar and tuple/list input on np.PyArray_MultiIterNew2

[zoj613]

For what it's worth, I ended up defining a function like:
cdef np.broadcast broadcast(object a, object b):
"""
Broadcast the inputs into a multiIterator object.
the input can be a scalar, list, tuple or numpy array or array_like
object.
"""
cdef bint is_a_seq = is_sequence(h)
cdef bint is_a_seq = is_sequence(z)

h = h if not is_a_seq else np.PyArray_FROM_OT(a, np.NPY_DOUBLE)
z = z if not is_b_seq else np.PyArray_FROM_OT(b, np.NPY_DOUBLE)

return np.PyArray_MultiIterNew2(a, b)

It seems to do exactly what I want. Not sure if there are any potential bugs
that could result from this approach.



--
Sent from: http://numpy-discussion.10968.n7.nabble.com/
___
NumPy-Discussion mailing list
NumPy-Discussion@python.org
https://mail.python.org/mailman/listinfo/numpy-discussion